Roy,
I'm going to have to feel my way on that. I'm going to try linen pressed over a mold and a lot of steam, as Tom describes, and also polyester and a heat gun. I've been browsing "permanent creases" and PVA and resins are suggested but would be a last resort.
The surrounds on the drivers I'm trying to fix look synthetic. They are awkward in having a wide half-round profile versus their diameter, I presume because they have very long throw magnets and so a lot of movement. The one has the original surround that has gone completely slack and the other has been replaced with foam which is stiff and doesn't let the cone to move much. I've had no luck finding foam surrounds that come anywhere near the dimensions and profile so am trying to get close to the original design and materials.
Steve