Neoprene varies in quality, as do most rubber products.  Some products
last a good amount of time and others harden or crack in just a short
time.  You can get a better quality sponge rubber that you will never
need to replace.  If the surface texture is critical, it may be a
problem to match it in a better material, but it is worth looking for.

E-A-R Specialty Composites makes sound deadening clay-filled foams
that are generally of very good quality, whether made of urethane or
other rubbers.  I have some very nice urethane stuff that I thought
for years was silicone because it was not like any other urethane I
have seen.  It has a very smooth, slightly pebbly, surface that seems
like it would make a good valve seal.

My material of choice would be silicone sponge which comes in a variety
of densities, pore sizes and surface finish.  McMaster Carr and MSC
Industrial both sell a variety.  If you use silicone, it will outlast
everything else including yourself.

There is a company in the San Francisco Bay Area that has a huge
assortment of sponges of all sorts, but you have to know what you want
to order it or else visit the place and be friendly to the salesman.
I don't want to give out the name to the general public for fear that
they might be swamped with annoying calls.  They are a wholesaler and
converter/manufacturer and cannot handle calls for a few dollars worth
of some obscure material.

If someone wants to act as a broker and buy a bunch of really neat
stuff to share with others, I'll give them the contact information.
They also have an amazing variety of wool felt, cork, etc. etc.

Best regards,
Spencer Chase
Garberville, Calif.
http://www.spencerserolls.com/