A lot seem to use a cut down cuff from a blood pressure meter, but since I new a longer thiner bladder I think to fit I was thinking about a length of bicycle inner tube. In theory the pressure differential should be zero apart from possible over inflation at the surface as far as I can see.
I don't want it too long or I can see the same stability problems as with a long thin water ballast tank, but removing the valve core and with and adapter it should connect quite nicely with the air pump.
Originally I intended to have a seperate air reciever but for simplicity I might just use the WTC which as it is large vs the bladder the change in pressure should not be too great. I'm a bit bothered that the silicon tube type control rod seals might act as a valve venting air out or that excess pressure might pop out the shaft seal (though it will have a keeper fitted) but I've not seen that mentioned as problems anywhere.
In theory as the WTC pressure increases the external pressure should also increase as the boat dives, but I doubt they'll keep in step!
Has anyone tried an inner tube as a bladder?