How would I make a clay house for my rats?

I am hoping to purchase two male rats sometime soon, but I've started getting things ready for them now. I've done plenty of research and am unable to find any measurements for a rat house. I am hoping to make one out of clay, (fired in the kiln and glazed - nontoxic) but am unsure of how big it should be. I want it to be able to fit both of them as they might want to snuggle! Thanks. :3