If you want ideas, here are a few pet items I have ordered for Syrian housing. I tend to go for heavier, wooden things that work well in a tank. I hope I'm allowed to link to Amazon.
1.
https://www.amazon.com/Kaytee-100079...dp/B0002AS5MI/
This one is a nice size, perfectly square (good for tanks), creates a level, is bottomless, and the top comes off. The only downside is that you have to assemble it yourself. When I ordered three of them, I ended up putting one together wrong, and another I ended up breaking a piece off of because it wasn't cut as well as the others. (I was still able to use it.) Otherwise, it's not a bad price for what you're getting. Just put it together carefully. I measured the roof of one of mine and it's about 7.5" x 8".
2.
https://www.amazon.com/Super-Pet-Lar...dp/B0002AS5KK/
I like this one too, for the same reasons, but it does have quite a foot print on it, and I have had hamsters reject it because of its height. It would be nice if they made it in a medium size.
3.
https://www.amazon.com/Grayline-4071...dp/B000LNVXIY/
I have made bedrooms for hamsters with small wire shelves like this one, paired with bricks, tiles, or boards. The hamsters love it. Just set the rack in the cage and set a tile on top of it. You can use a brick with holes for the door.