Update on progress
The design is looking good actually, I've tweaked the design a bit and ordered an M2.5 tap to cut a clean thread.
I'm not keen on paying out for loads of these until a few different people have given it a field test.
I've got 5 production models on order, but I might order some more next week - they're going to be £15 each (plus £1.50 p&p if posted out).
So... I'm looking for 5 testers