I rock Gargoyles Classic sun glasses, because they are the only ones I have found that I can tolerate the prescription inserts.  My problem is that the insert mount is a very small piece of hard rubber and they are disintegrating on me.  Gargoyles has discontinued them and I have contacted almost websites that "used to" carry them, all report out of stock.  I have been to 2 different eye doctors and called every prescription eyeglass lab I could find contact info for, and no joy.

In this pic I am needing to make the small t shaped item in the lower right corner.


I have tried tire rubber and an exacto knife, but I do not have a steady enough hand, and it came apart.


help

Look at some stuff called Sugru.

You might be able to mold it into the correct shape.

I use Oakley Prescription for sun glasses and shooting glasses. They work great although the polarization on the sunglasses really fucks up the depth.. Yet they block out the light so all is good i just cannot walk wearing them due to the polarization just messes me up.

Smooth-On has all the materials for making a mold and then casting using two part urethane.

Plenty of how-to videos on their site.  Link

find someone who has just bought a 3D printer, they'll be glad to take on the project for you.

These would be my 2 choices. I would try Sugru first since it'll be easier to work with. If you can't get it with that, I'd try making a mold and using urethane.