So I thought long and hard a few months ago about doing what you are proposing. What everyone says here is good advice. I decided to buy a new Canvas from bear creek canvas, $1100. Great people. I wasn’t willing to give up the sliding beds for hard sides. BUT, I had an idea that would cost about the same as a new canvas: Using foam board insulation with a light layer of fiberglass and epoxy resin. Kinda like a lightweight surfboard. You can buy cheap single pane windows and install them in after laying the glass. I thought about making this for over the sliding beds too, but here waterproofing would be a challenge. My workaround was to use long flat flexible pieces of plastic that would be glued on and act as waterproof hinges. Like I said, total cost would be at or greater than a new canvas so I didn’t do it. Good luck and if you decide to go for it please post pics!