I want to create 4 cover boards to place on top of my bathtub to allow it to double as a seat/daybed/exercise table. It is inspired by Duravit tub covers which cost a fortune (just under $2000 for 4 covers needed for a 66" tub), see http://www.duravit.com/website/homepage/products/product_overview/series/happy_d.us-en.html/p-583860/c-tubs/sp-1. The weight of a single Duravit covers is about 7lbs.

Low weight, strength over a span of ~25", and some dampness tolerance are the essential qualities. I am thinking of using 1/2 - 3/4" plywood inside a leather cover with padding. But I am unsure about the strength. Another idea was 3/4" trupan.

Is there anyone on this forum who can help me design for low weight, strength over a span of ~25", and some dampness tolerance? Help is much appreciated.