Here's a link to my write up for my "storage system". I went a little farther than just a tonneau cover, but I wanted to still be able to put the rear seat in, as well as have more storage options.
Although I am currently working on rebuilding it out of metal, the origional version was made of 3/4" plywood, and covered with bedliner. It held up fairly well that way for around 3 years, just a few nicks in the bedliner from throwing stuff on top of the tonneau section.
As far as weight carrying, I added a couple pieces of 1" angle iron to the under side of the tonneau section for reinforcement. The largest/heaviest thing I ever had on top of the tonneau was a garden tiller. it held that weight just fine.