I would suggest not adding anything else for at least a couple of weeks while Brian is adjusting to the change - but here are some shelves I know about.
I got some from Ranchouse cages. They come with good fittings so are easy to attach, but they can be a bit heavy the bigger they get. Mine ended up sloping down slightly. You can also get plastic corner shelves. I had a very nice wood corner shelf, that wasn't too heavy from Amazon in Germany. It's not cheap though. The shelf itself was only about £6 (converted from euros) but with postage it was approx £16! It is very nice though and well made. The ladder isn't included.
https://www.amazon.de/gp/product/B00...?ie=UTF8&psc=1
I had a long narrow shelf from Ranchhouse cages that was quite good for filling a gap plus it came in a pack with two "ledges". They are just a bit heavy that's all. They also do this really nice bendy bridge that can be used as a small shelf. It screws to the bars at one side and can be bent into different positions. I just used it as a ramp up to the Barney shelf, attached to the bars so it didn't slip.
They do shelf packs though so you can get a few in different shapes and sizes for about £15.
ranchhousecages | eBay
These are the plastic corner ones. They are quite big - 40cm each side.
The Savic one is also £15 - I reckon the German wood one is better value as it's so nice and easy to fit.
https://www.amazon.co.uk/d/Pet-Suppl...Savic+platform
This Little Friends one is about the same size and cheaper
https://www.amazon.co.uk/Little-Frie...NPY30Q1MFX22SC