Looking at building a hard top here in Costa Rica.

One idea I had was to build the frame/skeleton of the top out of
aluminum square tube welded together to form a grid. And then gluing
foam to the top, bottom, sides, shape it, skin it with fiberglass,
fair, prime and paint it. The wire chases would be PVC pipe.

So my questions are,

Is it a stupid idea?

If not, what kind of adhesive would be best to use to attach the foam
to the aluminum grid?

Is there a better material to make the grid out of?

What kind of foam board would be best to use for something like this?

How would one go about calculating the maximum cantilever that a
structure like that could span?

Thanks