Yeah, that's what I'm secretly hoping for if the Minda folks never step up: a Nash Metropolitan key, same as Lois Lane. I adore Mets, and have even been a welcome participant at a couple of their gatherings in upstate NY with my Amphicar. For some reason Mets are often confused with Amphicars. In fact the same Austin engine which pushes along the Mets was originally slated to power the Amphicar, as it did its prototypes, but they swapped to the Triumph Spitfire Mark I unit for production at the last moment owing to better supply commitments from Triumph.
To be honest, I could quite happily live without a locking gas cap. There's honestly no purpose to it, unless you also put a locking fuel tap on as well. I mean, if you just yanked off the hose there at the tap or simply cut the fuel hose, you could just drain the tank right there, with no need to go through the bother of siphoning from the fill hole. So, why have a lock there at all? If I cannot source a key blank, I might try to just rig it as elegantly as I can so that a screwdriver or even the ignition key can just open the damn thing.