I think it may be relevant to note that reproducing pianos generally
use around 30" to 35" water pressure (vacuum) whereas organs seldom
run at more than one-third of this, at most.  Duo-Art players used
heavy bellows cloth on their box pumps and 'steamboat' pumps, and many
of these are only just now needing replacement, after around 60 years.
I think the safest guide is to use the same material as the original
builder chose, as he had his reasons for choosing it.

Regarding the point about the corners of organ feeders being prone
to slight leakage as there is no cardboard stiffener at this point,
I have seen examples of feeders with a second layer of leather glued
inside the stiffeners and covering the corners to minimise any leakage
and we have used this technique with success in some re-builds.

Best wishes

David Evans - Lesley Evans Player Pianos, England