Thanks for the advice, Zizi66 and Cmd. Vyper!
Those paper buildings were designed to nest into each other for storage. If I added foamcore to the walls, I'd lose that storage feature.
I could try 1 or 2 mm thick card? That should
just fit. Or Just try heavier paper (I used 160g/m2 for these, it was the heaviest the local shops had available).
I'm still deciding how much effort to expend on the paper buildings though. They're really only intended as a stopgap until I get far enough along with my scratch-built structures. Which means, depending on the amount of hobby time I can muster, they could be needed for only a few months, or much, much longer.
Oh, and a question: Anyone have any advice on doing a roof?
I tried making one from card, with strips of tiles glued on. But despite pressing it all down with a fair bit of weight while drying it still warped to hell....
And I'd only done the first half of one side of the roof.