I had a good look on where you can buy the products (resin, hardener & fibreglass sheets), and found you can buy it all from Supercheap! Although not that cheap (its $12/pack for 1m x .5m of f/g sheets), you could buy all the materials for around $80. That doesnt include the final covering, but does give a clear cost of everything.
I want to hopefully do something like this in the coming months, as having the rear strut brace in the way means I cant properly sit a woofer in the car, and the added advantage of space saving has its perks
Cheers
Marc