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I wbuld build a frame from metal, flat bar stock at the base so you can bolt it down and remove it when you want to- then you can use whatever you want to construct the structure, even just a form that you can remove after the fiberglass has set up. Do you have experience with fiberglass layup? Sheet metal may heat up too much in the sun without some kind of insulation and a headliner, while fiberglass will not transmit the heat as much.
I think, if you have experience with fiberglass, you could do whatever you wanted- making it simple and boxy, or add water channels or even bulges into it to accomodate storage or speakers or something. Thin plywood like 1/4" is easy to work with, and would get sandwiched between layers of fiberglass, so it would be easy to make a structure, and you can get vent windows from RV supply shops.
I was just advising the metal or aluminum for the bottom area so that it will be able to be bolted down easily and removed when necessary, without the fiberglass being stressed and cracking around the hole areas.
I don't think there is anything published- maybe some people on the Hummer forums, but otherwise you are just going to need to measure out your vehicle, note any locations for attaching the top fo the body so some kind of reinforcement can be installed at that location, and then draw it out in a top, front, and side views with measurements. This will give you a reference when you build a frame to fiberglass over.