I put a plywood load floor over the rear seats in my R53 and covered it with a piece of carpet runner bought at Lowes. No hinge, quarter inch plywood. The seats are still under there ( I think! ) after 13 years. I stopped pretending anyone could get in the back seats on the way home from the dealer the day I bought it.
Fiberglass isn't flexible. It would break when you bend it back and forth.
That original package tray with the carpet hinge didn't support 150 lbs.
You need a piano hinge.
In reply to SVreX (Forum Supporter) :
i also was gogint ot say piano hinge,
for carpet. the black auto carpet at autozone (like for subwoofer boxes) is a perfect match to my r56.
i raised the floor in my car up to to the level of the seats folded down for a flat floor asn space for my toonz i used leftover 3/4 ply and black.
you might alos consider making 3 separate panels it woudl certaly be easie rto get in and out.
I used a 4'X8' sheet of fiberglass that is typically used in a shower/bath. One side had a texture that I sanded down the high spots but used the back side as the outer surface. I used spray adhesive to attach it to 1/2" foam insulation board. I covered the foam in self-adhesive felt. The fiberglass can be covered in carpet or vinyl. If you added the fiberglass sheet to the back to make it a sandwich it would be extremely strong.
4oz E-cloth will not work as a hinge. It'll tear after a few bends.
I did a trunk floor in my BMW with thin plywood and a big piano hinge then covered it in outdoor carpet. It's held up great to 500lbs of asphalt patch, a Volvo b20 and more.
What you'll want to build is a stressed-skin platform. Several ways you could make it.
How thick can it be? The thinner you go and still maintain the strength/light weight, the more expensive it will be. You can use an industrial structural foam like Diab, you can use Polyisocyanurate (or iso board), or you can just use lumber.
Diab is bloody expensive, but amazing. It's as light as styrofoam, but when you skin it properly it is stronger than an equal thickness of plywood and half the weight.
Iso board is still light, but not nearly as strong, but way cheaper.
You might also find that two layers of epoxy-bonded luan with glass and epoxy on the outside skins might do the trick.
The secret to a stressed-skin platform is to put two rigid sheets on different but parallel planes, bonded to structural framing (foam, lumber, etc) in the middle. In order for the platform to deform and bend, it must either compress the top level, or stretch the bottom level. For instance, if you just do 1x lumber under a single sheet of luan, the weakness is the framing. The framing can snap and deform relatively easily. If you skin the bottom of the framing as well, the entire frame becomes a simple weight transfer element. Since it is fully bonded to the bottom skin, it can't stretch or deform without stretching or compressing the luan.
I'd use 3/16" -1/4" plywood or melamine. Cut it so that there's a 1/4" gap between the panels and then glue the carpet to the panels.
Or use something like this:
https://monroeengineering.com/plastic-living-hinges.php
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