Furthermore, the techniques I'm using here are very simple.
I take this foam insulation:
Shop GreenGuard Extruded Polystyrene Foam Board Insulation (Common: .56-in x 4-ft x 8-ft; Actual: 0.56-in x 4-ft x 8-ft) at Lowes.com
It's wicked cheap (and you get a lot for the price) and you can cut it (knife, bandsaw, file, whatever), sand it, etc. I laminate a number of pieces together with Super 77 spray glue (give yourself about 24 hours to let that set up; you can certainly do it sooner though, read the instructions on the Super 77) to get a desired piece to work on depending on what I'm doing. Only thing to note is that both sides of this sheet have a very thin plastic backing ply that picks and peels off very easily; remove it as it causes issues when you start sanding on a laminated stack up (the plastic starts to poke through).
After I've shaped the piece, I then simply lay up a composite material (fiber glass or carbon fiber) with epoxy resin over the foam. This is key, it HAS to be epoxy resin as polyester resin will attack the foam and you'll quickly end up with a gooey mess. Yes, epoxy resin is the hard thing to acquire inasmuch as you can walk into any Lowes and buy polyester, but can't for epoxy. Got a West Marine near by? They should have it.
Once laid up and cured I then tend to remove the foam with acetone as it's easier to place and locate a hollow panel than one that is solid (which you then would have to clearance over all the bits and bobs). Deflash the part, trim for clearance (I use these for trimming all composites as they chuck into any dremel and do the job for just about any thickness composite:
Diamond Coated Rotary Cutting Discs 5 Pc), and mount.