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One could be made using a form for the fiberglass, and then, when removed, filled with foam. That is how boat hulls are produced.
Forms are made of wood, etc., with a fiberglass covering, then sanded smooth, and last, it has a gel coat applied, to make it slick, just like a reverse of what you want made. Next, a release agent is sprayed on or in the form, and then a gel coat over that, with glass applied next. Once it has cured, a jet of shop air is used to separate the the object from the mold, and then it is finished off.
In this case, you would want an internal mold, and lay the glass on the outside. Once it is cured, then you remove the internal form, and finish off the outside. The finish ought to be something similar to a gel coat, as it can be built up thick, and it is tough. Once done, fill it with foam.
You can add Kevlar cloth, too, and toughen it up more. However, the problem is, can you make it cheaper than you can buy it? A lot of the stuff to do this, comes in quantities that are way more than needed, and are expensive.
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