I do have the space, and I plan on using a CO2 chamber to euthanize the rats (I can rig up a basic chamber with items I have on hand, and a local grocery store sells dry ice that I'll use). I'm currently going back and forth between tubs and a rack, but I already have the space allocated.

Thanks for the tips. I've decided to pick up some rats at my local expo this Saturday, so I need to make a decision about housing them quick!