Ross,
The O-rings with a thin center membrane I mentioned earlier were like what you were describing. Not sure each would allow a cubic mm of displacement, but there were at least 4, maybe 6, per J-Box. In those, needed to put a wire in at least 2 fittings, which didn't leave much diaphram action available.
Yup, I know the ones you mean. And yeah, they don't allow much movement.
It's pouring rain outside, so I don't want to go topside and open boxes for a photo... so here's a rough mock-up.
An IP67 rated box. Seals tight... but summer to winter will almost always end up sucking something in if there's even the smallest hole. A standard nylon gland, a good quality party balloon. Missing from here is a filter. Cigarette filter, wad of scrunched up flyscreen, whatever is at hand to stuff up the middle of the gland to stop critters getting past.
The gland. Nothing special. Any hobby-electronics store or electrical wholesaler will have them.
Take off the end that closes it. This would normally be the "outside", although I will be using it the opposite way here.
Roll balloon up and put over the end of the gland. A tiewrap around it is a good idea to keep it there.
Drill appropriate size hole in case, somewhere the balloon won't interfere with your installed goodies.
Put gland in from the inside, nut on the outside, tighten. Usually these things have a rubber washer to ensure a good seal.
Poke filter material up hole firmly, so the air can pass but critters cannot.
I've used this in places it was nearly impossible to keep a box sealed and dry, and had years of perfect operation from them since. (Ideally, a second, sealable hole, and I blow in some inert dry gas to charge the box before completely sealing it)
