Hi there. I had virtually the same cage - it was the Savic Mickey 2XL which is identical to the Hamster Heaven except with smaller bar spacing. I actually bought one of the HH shelves for it as it didn't come with shelves - but ended up binning it! The one I had was the solid shelf - which has a ridiculous hump on it! Lots of reasons it didn't work but in the end the ladder broke and that was my excuse to get rid of it.
There are shelves you can buy, with all the fittings, in various sizes, and I'll link them below (Ranchouse cages on ebay sell them in all different shapes and sizes in kiln dried pine wood). There is also another one I like that I've used a few times which is less chunky but well made - it's only available from Amazon in Germany but I'll link that too - I like it a lot and find it's worth the extra postage cost.
But - I ended up setting up my cage without shelves - and instead of a shelf I had a large flat roofed house - the roof doubled up as a shelf. And that's when I found that Syrian hamsters really like large houses and tend to then move their toilet corner inside the house (ensuite bathroom) - so I then put the litter tray inside at one end - it worked very well.
However they still need to not feel too exposed from above when they are out of the cage, so I also had one smaller shelf over the house, and a hanging rat sputnik - which our hammy liked to sit under as well as inside.
The hanging rat sputniks work very well in the HH which isn't a particularly tall cage, so the hamster can actually climb into it from floor level - although they do need a way up to it as well.
Since then I have upgraded to a Barney cage and still have a large house for our Syrian - even larger now - it's a big labyrinth house and he loves it - has different rooms for nesting, hoarding and peeing in and a toilet/potty fits inside.
But you can do something similar in the HH. Rodipet houses are great, but expensive, but you can make a shoebox house which is the same thing and a good size. Cut the base out of the shoebox and keep the lid as a lift off roof (that way you don't need to take the house out so the nest doesn't fall apart) and you can just lift the litter tray out and empty it every few days. Cut a hole at one end of the long side - if you put a bendy bridge tunnel over the door it makes it dark inside and they really like that - they need somewhere dark to retreat to. The bendy bridge also acts as a ramp up to the house roof/level.
I'll add some photos of the way I had that cage, for ideas. So before you go spending lots of money on shelves, have a think about what would work.
Personally I would go for a good sized flat roofed house, plus a small corner shelf over it (something to sit under and climb on) at one end and something like the rat sputnik at the other end.
I assume you've got rid of the plastic tubes and penthouse as well? They are a total pain and don't help with normal toiletting and nesting habits at all. They do much better with a house sat on top of the substrate that's open underneath, so they can bury hoards under the nest and it stays cleaner.
So I'd look at houses first - and then you'll only need one smaller shelf.
This house by rodipet is about the same size as the one in my first photo, but much better - it has a lift-off roof in two sections and one section is a rough granite tile - so it's ideal for putting a food bowl on and shortening their nails every time they go to eat (their nails can get quite overgrown unless there is something rough to keep them trim).
https://www.rodipet.co.uk/houses/rod...t-rodents.html
It fits along one end and still leaves plenty of floor space, plus doubles up as a shelf.
And you can get legs for it
The house sits on the legs, which raises it so it is level with the top of the substrate. That makes it stable and safe and they can tunnel under it or bury things down under inside. It also means the house roof is the right level for a shelf
https://www.rodipet.co.uk/houses/rod...of-houses.html
However it isn't cheap. And a shoebox house would do just as well.
If you did want a shelf as well, this is the one I like:
It works out about £17 with postage but I found it just perfect - makes a good sized shelf, not bulky, very well made, comes with all the bar fittings, and doesn't take up too much space in the cage - ie not too big, not too small.
https://www.amazon.de/gp/product/B00...?ie=UTF8&psc=1
These are the ranchouse cage shelves. I have had these but find they are quite weighty, even when fixed to the bars - they're also a bit slithery on top for some reason, but they're fine really. The smaller ones are better - the larger ones didn't sit up straight due to the weight, if only attached on two sides.
However I found it hard to find exactly the right size and shape which is why I got the one from German Amazon. With wood shelves and houses it's best to paint them with plasti-kote petsafe waterproofing paint - very quick and easy - 10 minutes to paint a coat on - it smells of nothing really - and a few hours to dry between the two coats - then they're wipe clean.
I actually haven't plastikoted my shelves but put hemp mat on them instead - and our hamster has never pee'd on the shelves - but inside the house you need to do it in case they miss the toilet and it gets pee-stained - with the plastikote they're wipe clean inside.
If you just want a small shelf these are good - they are bendy bridges that screw to the bars in any shape you want. I use one as a ramp to a shelf at the moment as it's secure and doesn't fall down or sink, being screwed to the bars.
https://www.ebay.co.uk/itm/BRIDGE-HO...sAAOSw5VFWHRqJ
These are some of the ranchouse cages shelves
https://www.ebay.co.uk/sch/m.html?_o...shelf&_sacat=0
This is the German Amazon corner shelf in our Syrian's cage - over his house roof (this cage is taller than yours so the shelf is quite high up) and the bendy bridge ramp/shelf.
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Below is the old cage set up in the cage the same as the Hamster Heaven
On the second photo I added a rat tube tied to the roof, leading between the house roof and the sputnik, so that made a kind of roof run and added to variety and things to do - but it looks a bit cluttered! On the third photo I had got an even bigger flat roofed house - but that did take up too much room in the cage but I was upgrading him to a bigger cage shortly after that anyway.
I didn't know about extra shelves available then so just had a little lava ledge - which you might be able to see - back right corner - and covered the gap where the house roof finished. And he loved sitting under that little ledge.
They need a shelf, or something overhead, to sit under to feel secure.
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