Bottom is 1/8” or thicker aluminum with foam pad. Top is 1/16” or thinner plastic sheet with thinner foam pad. It formed a curve somewhat to your back.
if you are dead set on making your own rig - your really only have two major issues to consider - temperature and if you’re packing out animals.
if you’ll ever hunt with it above 60* on a regular occasion, go with the molle frame and mesh(or no mesh, i cut mine off of my hybrid). If it’s always cold where you hunt, the solid back of plywood or plastic and some foam and fabric will keep you warmer(and be 10/10 on comfort instead of 9.9/10 of molle frame).
if you’ll pack deer, make it out of the molle frame. If not, it doesn’t matter.
Also, adjustable fork, and adjustable side webbing for the angle of the seat are non negotiable in my opinion for this type of seat. Otherwise, buy a rock harness, make a one hair tree seat, and use a golf bag sling for a back rest.