This is the types of neoprene that McMaster has:
High-Strength Multipurpose Neoprene Rubber
Offers an ideal balance of increased strength, and good resistance to oil, flame, weather, and abrasion even with rough handling.
Commercial-Strength Neoprene Rubber
An excellent all-purpose rubber that maintains its resiliency even after extended compression. It also has good resistance to weather, alkalies, acids, and abrasion even with rough handling. Moderate resistance to oils. Also known as chloroprene.
Ultra-Strength Neoprene Rubber
A virtually pure neoprene, offering an excellent combination of strength and oil resistance as well as good flame resistance.
Fabric-Reinforced Neoprene Rubber
Fabric reinforcement increases strength and durability. Ideal for stress applications such as gasketing and diaphragm packing.
Antistatic Neoprene Rubber
This high-strength, flame-resistant neoprene conducts static electricity and protects equipment from damaging static charges, vibration, and shock.
Neoprene Spring Rubber
A hard, resilient material that bends but resists breaking.
Neoprene Cork Blend
Cork granules blended with Neoprene rubber make an excellent vibration-dampening and cushioning material. Compressibility is similar to that of natural cork. Resists some alkalies, salts, water, and some detergents.
I think that the Fabric Neoprene rubber would be the best because it mentions gasket material, which has been used as sealing face before.
Also hubb made a topic that mentioned that flutes use a layer of fabric under the sealing parts of the keys of flutes which makes them seal better so he thought that same concept can be applied with pistons and better sealing.
Also, my barrel is 3 inch diameter, so what thickness should i get?
EDIT: Also, what would be better, medium soft, medium hard, or hard?