RCS Holster Wedge
Designed to tip the rear sights into your body.
Made by Raven Concealment Systems (RCS), this wedge is made from a moderately dense rubber and attaches with two posts/screws. The hardware is supplied with the wedge. It was designed to fit the RCS Eidolon, and I cannot guarantee that you can make it work with any other holster. You will need to select whether it's for a left or right handed holster as it's not ambidextrous.
This wedge is optimized for appendix carry
If you want a closed cell foam wedge that is universal, you can find that here.