It's not rigid, it be kind of like a firm/hard finger insert.
I didn't really like it, it felt slick on my thumb , plus would slide down in the hole and I didn't want to glue it in place (I figured you'd destroy it trying get it back out if you glued it)
Here's mine, I still have it after all these years (Proshop gave it to me to test).
Here it is started into the hole of my thumb insert. (It fits better in it than in a milled oval hole)
you can see how the V notch is squeezed together, that's what hold's it in.

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