A former friend of mine has built two giant rat playgrounds/cages out of two old cupboards, with holes between the levels and many climbing possibilities, the doors were cut out and filled with a wire network, so that you still can use the original hinges and can open the whole thing up. There were always about 4-5 rats in each one, if some didn't get along well, they have been put into the other one. All of them were saved, not one bought.
I couldn't cope with the fact that they die so fast though.
She has a pug now instead... I liked the rats way more.