I wonder how much the surround really changes the Thiele-Small parameters.
For a repair job on some cheap speakers, I used Saran wrap, which is made of vinyl or a vinyl compound. It looked like hell, but it worked okay.
If you take the surround off the speaker, it droops limply in your hand, as does the brand new foam replacements. Yes, I know the circular shape, when installed, adds stiffness.
But it seems to me that 99% of the stiffness in the suspension comes from the spider. If you cut the spider out of an old speaker, it certainly is not limp-it's springy.
I think the surround is just stiff enough to hold the cone in the center, and is so soft compared to the spider that the surround material matters little. I think it is the spider that is the key to the Thiele-Small parameters.
I have never measured the parameters to make sure, but that is my educated guess.
By the way, if you ever do decide to "do-it-yourself" without a kit on some cheap speakers, use Saran or vinyl wrap only. Never even try to use the normal plastic food wrap. Why? Nothing sticks to it. There is no glue that sticks to that polyethylene wrap. But most any plastic glue will stick to Saran or vinyl wrap. As long as the substance has "vinyl" in the name somewhere-like polyvinyl chloride, (PVC), the plastic or vinyl glue will stick to it.