I found it but it doesn't make any sense to me now lol as I can't remember what all my coloured lines were on the graph paper!
I had two options. One was - have a shelf along both sides and another shelf along the back a bit higher up, overlapping the side shelves, with the wheel under the back shelf. And hemp mats on the shelves so any falls from the roof wouldn't mean landing on a hard edge of a shelf on the way down. And having most toys under the shelves.
The other one was having one side of the cage with cardboard or perspex about half way up (eg along one side and halfway along the front and back ish). And having very deep substrate at that end, and a shelf at the other end. A large cork tunnel next to the shelf (to lower fall height again and give a ramp up).
What I tried to do in Newtie's Barney cage was increase the floor height at one end by using the Rodipet shelf with the deep sided wave bath on top. It meant there was space under the shelf but the substrate was much higher up in the wave bath and could be banked up next to it so the substrate didn't fall out. It looked good but what I didn't bank on was Newtie deciding to nest under that platform which was really low and inaccessible! So I wouldn't recommend that.
It's getting shelves and getting them the right size that's the tricky bit.
If you went with my option 2 above it would be easier than having a full level made/fitted. Eg have a 50cm deep shelf made that covers half the cage and have the other half as deep substrate.
Or - those white corner shelves you have - more of those. It's quite a cheap option - I think they're about £6 each on Amazon. One in each corner would cover the depth of the cage and 80cm of the length. Then you would just need some smaller shelves to cover the open 20cm front and back. That would still leave a hole in the middle so that would need thinking about.
Or another idea I had was to just block off the roof! If you have the big front opening door, you don't really need the door on top. If the roof is blocked off then they can't fall from the roof. I sort of did that in the Barney for Charlie when he got old. I moved all the shelves up to roof level so effectively it was mostly a wood roof. A couple of mm from the top so no way a hamster could squeeze between the shelves and roof.
A bit of a waste of shelves possibly, but if you had the roof bars effectively lined inside with wood or other types of shelves and no fall risks, then you could set the rest of the cage up how you like
And not worry about the height.
That's almost the same as getting a full level made.
Not sure any of that is much help!