SO3 XXL - A noob requesting help with sheet metal cutting

When I used HDPE it was a section of a cutting board on top of the MDF.

Menards has 1/4" thick HDPE: https://www.menards.com/main/building-materials/panel-products/specialty-panels/plastic/1-4-x-4-x-8-hdpe-panel/p-1444424094192-c-14048.htm — a sign shop ought to be able to get you thicker pieces reasonably affordably.

I wonder if we aren’t on the wrong track with all this — why not just build a torsion box and a frame and pour in epoxy?