Mine is looking in similar condition... but grey and falling to bits.
At some point I'll take my rear shelf off, install some new speakers and fix up the carpet/cover. I might take a shot a designing a replacement, but would have to figure out some heat resistant plastic to make it in, I'm sure PLA would quickly deform as it's hot on that shelf. Possible shapeways could print one in nylon, which could painted to match the interior.
Good luck, let me know if you find a grey one.
Cheers,
Ian.