I was super intrigued by the soundpath feet when I was shopping for subs. As I said before, they do work. 50 clams, retail for a 4-pack is a little pricey, but considering many other solutions are much more expensive, not a bad option.
As best I’ve found, most threaded inserts being used for these will be covered by the hardware kit included with the feet: m6, m8, 1/4-30, and I think m10, too.
That said, if it is a weird size, just take the existing foot you remove from your sub and visit a good hardware store. You should be able to identify the thread very easily... but likely that shouldn’t be necessary.
Assuming of course you choose this option.