Mycroft, the Witness magazine seal is made from PTFE/TEFLON which is not suitable for this type of application.It makes a good seal on a permanent joint, but because it is malleable and deforms is not suitable for a seal that is continually compressed and released, and will deform.The best material is polyurethane which is very resilient and hard wearing.I think Nick of magic9design can help you.
http://www.magic9designltd.com/
Baz