I've been very happy using aircraft plywood for stiffeners. It comes
in thicknesses from about 0.75 mm up; for larger pressure pumps I use
1.5 or 2 mm thick material.

It's quite stiff, strong, void free and made with waterproof glue.
There's no reason to think it won't last a very long time. You can cut
it neatly with a saw, laser, or CNC, or a knife for the thinner stuff;
scissors work on the .75 mm. I sand the edges round and round the sharp
points so that it won't cut the covering material; it takes a few
seconds by hand.

It's wood, so all your normal glues will bond to it. I use hot hide
glue for convenience, you can just iron it to your leather if you get
a bubble. Not having to clamp to get a good glue joint is very nice.
I quit using rubber cloth a decade ago and really don't miss it.

Roger Wiegand
Wayland, Massachusetts, USA

Attachments:
 [ https://www.mmdigest.com/Attachments/26/08/01/260801_124225_IMG_3351.jpeg