Closed cell foam has worked well for me for piccolos and band flutes with smaller keys and flatter cups.
On large salt spoon keys with deeper cups what do people “back” the foam with?
I know Casey and Rod Cameron recommended ca glue/ baking soda to make a hard “marble”
then sand down to have a “flat” surface to fit the pad on.
what do people use to be less permanent?
one thing about foam is its cheap. Very cheap.