Interior Hollow Core Doors

This idea may be suitable for other scales; I just model in HO.
50 year old houses often had flat hollow core doors inside, often called Lauan doors for their veneer. The bathroom doors were often 24" wide, the standard depth of a module. Unless it has already been cut down, a door will make 2 double modules, or a quad if you you can manage it with all your scenery. These doors can often be found at Habitat for Humanity ReStores, Craigs List, Facebook marketplace, etc. for cheap. Add some 1/4" plywood strips for a facia and some leveling screws of some fashion. The trickiest part if you aren't good in the wood shop is replacing the wood in the resulting hollow space after cutting the door down to the size you want.