Woody wrote:

If you get the foam from Wilmslow Audio (who are in Hinckley or
Nuneaton or somewhere like that although they started life in
Wilmslow) get some Tygan grille material as well. You will probably
have to take the existing material off to fit the foam, and Tygan
shrinks under heating (hair dryer) to give you a nicely tensioned
appearance.

I don't follow why I'd need grille cloth as well as foam (originally,
there was just the foam).

They look like
http://www.audiocostruzioni.com/r_s/diffusori/diffusori-5/royd-audio-conistor/constriction%20r.jpg.

Daniele