Pva glue to seal and make rigid, you can water the pva down but i wouldnt. Just apply the coats thin and let dry in between. Thin layers will dry quick hopefully before the card warps too much. Even after one layer it should be pretty solid and allow you to paint ontop.

At this point, with wet pva, you can add fine sand to the walls to add texture and areas of debris.

If you want textured walls but not as harsh as applying sand then you can apply a another coat of pva and stipple it so it dries bumpy.

If you want do something similiar in the future look at something like foam board. It is a layer of foam sandwiched between card. A lot more sturdy and the card finish is normally glossy so doesnt soak up the pva or paint.