Yeah... not an easy project. The box itself is totally different. You can't just flip the left side around, it would have to be turned inside-out to work.
I think the easiest solution might be to cut the vent box out of a complete, right-hand heater box assembly and graft it on to a left side box. There is an overlapping seam on the bottom secured with brass rivets so that's kind of how the factory made them. I'm sure they didn't want to make the tooling for something like that since it was a non-standard option limited to year-round warm climate markets.
I could never figure out exactly what material was used for the heater and vent boxes though. It's similar to fiberglass and may very well be some sort of pigmented resin/mat type stuff.