That is gen-u-ine ABS plastic. A friend got it for me from where he works, I don't even know where it might be available retail.
At some point, I forget the title of the post, Serban asked me if I thought Plexiglas would work, and I don't see why not; just make sure it's thick enough (1/4-3/8") to support whatever you might have back there. They are also making sheets of PVC now (the pipe stuff, ya know) in varying thicknesses, although again I'm not sure where to get it. Home Depot or Lowe's might have it, or at least know where to get it. Call before going...
Smooth, my man, I had forgotten that you were in on this subject as well. Please don't put a hit out on me. I guess you can keep your trunk the way it is; as for me, I need to put stuff back there.

I don't think my saxes would like sliding around on the bare metal, considering that they're relatively fragile and are worth more than the car.