Yeah I hear ya about wanting to practice with foam and fiberglass by making something simple first like a tonneau.
Also for what it's worth the tonneau that I have now is also a fiberglass/foam composite. It was manufactured by a company called Checkmate. The foam that they used in it's construction is the same bead style foam that you are using and the core is only about an inch thick. I have no idea if they used any coating on it or what kind of resin they used but I just thought I would throw that out there. Also for what it's worth considering it's very lightweight construction it is actually very strong.
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-Kevin
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