The rubberized cloth used to re-cover the bellows of roller organs
and organettes is typically .015" to .018" in thickness, cotton cloth
two sides with a rubber sheet center.  It's not advisable to use cloth
above this thickness for the pumper bellows as it can create binding
and clearance problems in some organettes.

Thicker cloth can be used on the reservoirs only.  Thinner material
such as .012" to 014" thickness can be used for pumper bellows but
would probably require the addition of card-stock stiffeners to keep
the bellows from blowing outward.  This is commonly found on pressure-
operated organettes or cylinder music boxes with organ attachments.

Harold Draper