You might be overthinking this.
Really depends on the complexity of your tuning table design & set-up, but:
1) For blowing single reeds and reedblocks I just have small melodeon bellows with a plywood top with a hole in it. No gasket. Doesn't stop me from tuning. Final tuning stages are done inside the box anyway.
This is all I use (small, portable and does the job) and I see little advantage in having anything bigger or more complex.
I started off with window seal material commonly used for bellows gasket, but with time the glue has weakened and I got rid of the gasket.
Car wash chamois glued to ply will do better than you'll ever need.
2) A great Idea that I saw at a professional tuner's shop was to have bigger bellows with a plywood top large enough to mount an entire accordion half-body on it. This way you can very quickly take the readings from the reeds "inside the box".
He used towels for gasket, so again, no need to hunt down the thoroughbred Persian goats with the softest skin on Earth.
I've longed for one of these for a while, even thought I do not do enough tuning to justify having one. Since you got me thinking about it, I've got spare bellows right now, so might finally build it.