Another option would be to use foamcore cardboard in lieu of plywood in a traditional cookie cutter design. The foamcore does need more support (shorter spans between supports) than same thckness of plywood, of course. But if you start with a foam base, you can cut blocks of foam and put as many supports in as you feel necessary, w/o adding significant weight or work.
I made a small table-top over/under Christmas tree layout using 1/4" foamcore, double thickness for a base, single thickness for the subroadbed, and supports were made out of squares of spare foamcore (would have been easier/quicker to use blocks of foam). Scenery was made from extra pieces of foam that was around (some of those "florist foam" blocks, some fitted packing foam, etc), and covered with spackling compound. It ended up to be straightforward, easy, no mess, light, stiff, and has survived being stuck awkwardly in a box for several interstate moves. Most importantly at the time, it did not require power tools of any kind.
---jps