Big cardboard box?
Combine pieces of cardboard for a bigger box?
Cover with window screen, chicken wire, sections of wire dog crate, metal grate borrowed from a grill, mesh child-gate or dog-gate, etc. Possibly even cover part of it with an old sheet, although that's not nearly as good for ventilation.
I just had an idea. What about putting them in a four to five person human tent indoors? Its as hillbilly as can be but would there be any harm in that?
Yes, that might work. Of course the tent will get dirty, and it is not very strong (against a determined dog or cat), but as long as you don't have housepets that will try to get in, it will probably work fine.
I've also made a big brooder in a garage:
Tarp on the floor (to protect the floor), with cardboard on top of that, and bedding on top of the cardboard.
Surrounded by a combination of dog kennel pieces, child-gates, and pieces of cardboard.
Covered by more such things, including some chicken wire.
It worked well enough for as long as I needed it, and then I took it all apart again. Some things went back to their original uses after a cleaning, and I disposed of the other parts.
