Fuggit Khan:
I like vacuform stuff…I’ve built lots of various vacuform kits and terrain pieces over the years. To give it strength (and it would work great for this step pyramid) I use plastic spray foam insulation. Comes in a shake and rattle can, and easily sprayed to fill the inside of any vacuform molding. The stuff is light weight and dries hard, makes a very solid and sturdy model once dry. Example of the stuff here: