Props Making SD diorama

Kappa/foam core board/poster board is often used for architectural models and miniature stage design, it'll be absolutely fine for doll dioramas. 1cm thick board will not bend a lot but you might want some extra support nevertheless, just some slats of thin wood for the edges maybe? I've used the material a lot and it's really fun and easy to use though!

Here's a bad drawing of how I'd reinforce a large wall from the back:



You can just glue or tape the wood on.

 

It's strong enough to stand and fit together. But I noticed that it can warp if it gets wet, if you try to paint it, or put wallpaper on it. It also tend to deteriorate faster then other materials. Awhile back I built a 1/4 diorama using it at the base. I ended up having to throw it out as it didn't hold up.

I found out that those tri-fold cardboard display boards actually worked and held up better then the foam boards, unfortunately they are not tall enough for the 1/3 scale. You might try finding other thicker cardboard like those tri boards that could work. I have had some of these walls for years, that I still pull them out for my 1/4 dolls.

What I use now is photography backdrops for my walls. You only need to get the small ones at about 3ft in height for the SD and even the uncle sized dolls, and can find some very nice walls for around $10. The reason I now prefer the backdrops is because most of my dolls are 1/3 and up, the diorama is just too big for me to keep out full time. This way when I am done with pictures I can put everything up neatly.

 

This lady on Youtube has great info about building sturdily with foamboard: She's built a huge house out of it and her methods are designed to last for a long time. She has other videos about wallpapering foamboard etc, and is really great about answering questions. Definitely check her out if you're planning on building with foamboard!

 

There's a room box tutorial on youtube that uses thicker rigid insulation foam boards (not craft foam board) which would likely last much longer and be stronger. You can get them in very large sizes, so it should work for 1/3 (here's a forum thread following this tutorial for 1/3, actually)
the vid tutorial:

(if it says vid unailable, here's a direct link)

 

The type of insulation foam board used in the video and thread (XPS foam board, like seen here) is non toxic to touch and cut, if that's what you meant. The one you'd be looking for is pink. It's "extruded polystrene". The only guideline given is not to cut it with heat tools (like hot wire cutter) because that creates gas, but just cutting it with a normal cutting knife is fine.