And here is a link to the inserts we used: https://www.mcmaster.com/94669A362/
If I were to make another, I'd be inclined to make the mounting flange a bit thinner (13mm instead of 15mm) so that the metal inserts stand proud on the flat side and fill the little recesses in the car's flange. That way, the bolts would be compressing the inserts and not the plastic. Here is a modified version with the thinner flange:
