The TNW extended rail foreends I've seen have a sliding key lock that locks the picatinny rail sections together and prevents it from turning.
Ah, didn't know that. Definitely fixes the problem, more or less in the same way I figured out just by a different mechanism. Thanks for pointing that out. Guess I could have added a bit of pic rail to my shroud and set up a lock there as well, though I suppose TNW's solution allows for adjustment of timing which would be a bit beyond my abilities.