"But, there are simple, either free or very cheap, stand-alone utilities that will create double-sided meshes for you from an original .obj. [...] I used to have one, but it's on an old, dead, hard-drive at the moment."

You're probably thinking of Objaction Twoface, by Maz, who appears to be no longer around. His Mover and Scaler are popular downloads which have been preserved elsewhere, but as 3dcheapskate notes, double-sided zero-thickness meshes became persona non grata with the advent of Firefly and I don't think anybody's bothered mirroring it. I can reunite you with it if you really want. :)