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: — 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?