My answer to the cork or leather question is "neither".  I use closed
cell polyethylene sponge.  It comes with a carrier (thin stable Mylar
backing) and a transfer adhesive.  It compresses to form a perfect
seal, costs almost nothing, is easy to cut with a scalpel after
installation, is easy to remove and will last about 1000 years.  It
is non-reactive and will not cause anything to corrode nor will it
migrate.  It will not absorb water.  It will not swell or rot.  It
comes in various thicknesses.  The only complication is that it will
change critical spacing if you crank down on it so you have to provide
little stopper spacers to maintain the proper gap.

The stuff that I use is white but you can paint the seal line (with
a sharpie) after installing it.  It may come in black also.  Only buy a
good grade with an adhesive that will last as long as the polyethylene.
Acrylic or silicone is the best adhesive and, "no," a silicone transfer
adhesive will not migrate into the wood.  If you can't get it with
acrylic adhesive, you can use 3M VHB transfer tape on plain stuff.  To
make gaskets for an Ampico, you just cut a rectangle, stick it on and
cut out the airways with a scalpel or "X-acto" knife.  You don't have
to tighten the springs to the point of near collapse to get a good
seal.

The best thing about polyethylene is that is compresses without oozing
out as do more resilient sponges.  This allows it to seal around small
holes without occluding them.

Best regards,
Spencer Chase