Last night I glued corrugated cardboard rectangles inside two of the plastic crates then I glued the crates together to make one apartment building. I used Aleen's Tacky Glue which seems to have bonded fairly well. I would suggest a tacky craft glue over a regular PVA glue or perhaps some wood glue for this. The cardboard will provide a base for me to glue details on like doors, windows, and tech bits. Normally I would try and make a building like this playable and use clear plastic for windows and add floors and doors but this is suppose to be a simple and cheap terrain project aimed at new players so I decided that would be too complicated.