RE: question about norvels and rubber
JM:
I think you mean "Tetra" on the bubble less tanks.
To make your own you can use surgical rubber gloves, they deteriorate just the same as your surgical tubing pressure bladders though, shouldn't run one more than a month or two without changing it.
You can also use the disposable polyethylene gloves, they have an indefinite life but they are more trouble to seal the the fittings inside the tank.
With either material you have to give the clamping screw of the plug and the vent pipe end extra care, as they can puncture the internal bladder if not carefully rounded on the ends.
Bill.