I've had a similar idea, since at the moment I use a block glued to the bottom plate to keep the pro micro/trrs jack from moving. There's
this build which has a separate piece for the connectors which is then screwed in from the bottom.
However something that clamps or is a friction fit with the front wall so that it keeps the pro micro/trrs jack from moving but is easily removable would be ideal to me (somewhat like
this). Whenever I get around to it I'll see if I can design a holder & modify the case to accommodate it. I'll likely post it in the dactyl-manuform thread if it works out.