Sorry,
Finally got around to this:
Mark's right in the 15mm x 15mm measurement.
It looks to be some sort of high density foam, bit denser than the typical interior panel anti rattle stuff.
Flat bottom, so anything of similar dimension with a self adhesive backing will do the job.
Theres nothing fancy in the material or how it's put together in the genuine factory replacement. Neoprene or some sort of generalised rubber seal might actually be a better product here I think.
Three or so years old:
