I ended up DIYing a rear plate out of 1/2" Baltic birch. It was 49 bucks and a few hours of time. I get the convenience of just bolting a GG in, but 650+ bucks is insane. Their stuff is just 1/2" Baltic birch with bed liner sprayed on it, fer chrissakes.
I'd liked to have made a rear plate out of polypropylene honeycomb. Econcore supplies this honeycomb to many manufacturers of finished panels like Plascore, Renolit, Thermhex, etc. This would be the lightest way to go, and it'd be totally immune to moisture. Some panel makers like Renolit add a grippy textured surface too.
Adding threaded inserts to honeycomb panels gets more complicated (but not impossible) though.
Too bad Airstream got an exclusivity deal on TransCore panels made by Ridge Corp. This stuff is a wood substitute panel made with a core of Nomaco HercuLean (extruded structural foam) that has the screw retention properties of wood. Maybe Nomaco sells HercuLean to other panel makers?
Probably a lot work to save a couple dozen pounds, but it grates me to expend payload capacity on a cargo room floor plate...