Teflon, rubber and similar synthetic materials are closed, non-porous materials that can be used as upholstery. To give them the necessary strength and straightness, the material is often glued to metal plates.
These materials can seal well and be durable. Depending on the instrument and preferred sound, they are quite interesting and are worth considering.
Compared to cork and leather, the desired resonance is often lacking. Teflon pads can become sticky. Due to the high sulfur content, rubber pads can cause the silver to tarnish more.