I am glad this How To is here. I am working on a rear corner box much like the one described by the author but mine goes all the way to the trunk floor. I did the masking tape and aluminum foil deal but also used foam core board to create the lower half.
Tonight I laid the first layer to make the shell fiberglass and pulled it from the car after it was dry. After pulling off the carpet corner, tape and foam core, it looks paper thin but seems like a good start.
Tomorrow I am picking up the MDF, fabric and dowels to make the ring and front with but have a long way to go before I need that stuff.
As for material supply, I bought the resin at West Marine as suggested but got the glass at TAP plastics because it was cheaper and had a better selection. I also got some BaltekMat ( see
http://www.tapplastics.com/shop/product.php?pid=93& ) it should work well to cover the flatter sections for reenforcement. The demo piece they had was quite light but very stiff once set up.
I'll post my progress and pics in a few days.
Chris