I read all the suggestions and thought of something that might help.
I am not building one of my own, but will purchase one from one of you
guys when we decide which one is best, most versatile and reasonably
priced.

If you are going to read pneumatically using pouches, make them as
small and light as possible and use the moving magnet on the pouch,
but use a vacuum-sealed reed switch like those used in electronic
organ pedals (Rodgers).  They are also used in every SAM or Stop
Action Magnet made by Syndyne for your theater organ whale's teeth
tabs or tilting tablets.

The switch contacts are only a few microns apart, are sealed in a
vacuum tube and the whole thing is an inch long and thinner than a
pencil lead so it can be soldered onto a board.  Since there is no air
around the contacts it cannot arc, so no burning occurs and contacts
last forever.

Just a thought!

D. L. Bullock -- Piano World -- St. Louis