As part of my trailer design projects, I designed and made molds for a series of framing parts that make it easy to assemble a very professional looking camper top for Jeep-tub trailers. Here are a few of the parts:
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There's a special set of parts for making teardrops, including framing for the curved rear hatch to make one of the more difficult parts of making a teardrop easy.
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The parts can be used for simpler things, like this hinged lid:
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The same lid/pop-up canvas is used in this larger camper:
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These parts haven't made it to the market although I've still got all the molds.