Try googling for something like "fiberglass subwoofer" and "enclosure, cabinet....etc"

I think for mere mortals without access to vacuum molding factories, one of the most common materials for this kind of thing is some combination of plywood, MDF and polyester resins. When I built a custom subwoofer cabinet I found some decent HOW-TO examples on the web - folks that molded polyester resin into trunks, tirewells, etc. This isn't the same as what you hope to do, but some of the techniques can be the same.

Using fiberglass/polyester with just a few wood braces and inserts will give you something lighter than if you use a lot of plywood.

Polyester resins are used with chopped-glass mat. Aadvantages of choped glass are that it is easy to mold and, unlike woven fiberglass reinforcing sheets. it can be sanded smooth (woven will always shoot out glass strands when sanded). You can either lay it up over a mockup that you build or inside a form that you build. You'll need to use something as a "release agent" that allows you to separate the piece from the mold. Once you get a basic casting done, you can take it somewhere and work on adding braces, sanding. If going the polyester route, make sure to finish all surfaces with a sealing coat (forget the name of the stuff they add to do this) regardless of whether you are going to paint of carpet it -- otherwise your work will stink forever.

There's a great fiberglass store in my neighborhood that has some HOW-TOs on their web site that might help:

www.fiberlay.com

They also sell mail-order, I think, and are the best, I think, at what they do - "Resins R Us" Final advice: go to a tool or paint shop and buy a whole box of disposable rubber gloves!