Cardboard

Cheap and plentiful material. I believe by laminating layers with opposing grain into "plycard" it will be very strong.

I'd like to try finishing the final product in paper. Paper is cheap and should look fine painted. Though with the MDF boxes, getting it flat was a challenge. It may not be as bad here because the paper was structural there which made it hard to work with when wet.

I'd also like to try wood veneer and see if I can make something that looks nice.

My initial test was just 3 squares of cardboard with alternating grain. I used cheap, clear school-glue. The white glue is cheaper, so I should try that next time I need to purchase glue. I diluted it down about 2p glue to 1p water. The right amount of glue seemed to be stripes about 2" apart. The outer layers of cardboard were unevenly wet and wanted to warp, so they needed to be held down.