Progress of a sort!
I contacted Rubber the Right Way (
http://www.rubbertherightway.com/), and asked if they would be willing to reproduce these given a usable template (they also offer flat material in varying thicknesses, so making my own from their stock would also have been a possibility). Robert got right back to me, and asked me to send a housing and an existing gasket. They would be happy to add these to their catalog, it seems. Turnaround is approx 4 weeks.
I just so happen to have a spare pair of backup lights complete with gaskets lenses and bulbs, so I will be sending one off to them at the beginning of next week, when I'm back home from this trip.
Sometimes, you just live right, I guess. :-)
I'll update the thread as we move through this process.