As the others have mentioned a shoebox makes a wonderful roomy house for hams. As it's open underneath you can put in a nice thick layer of substrate and plain loo roll which they can snuggle down into on cold nights. The size of the box also means you can put the potty inside on the opposite end to his nest (like a little ensuite!).
I use this setup for my hamster and I think he appreciates not having to wander too far when he's sleepy, so he almost always uses the potty
