Get some 250 grit sand paper, 400 grit sandpaper, and a sharpie. Wrap the sandpaper around the sharpie and shape the mag relief as you want. I do this for creating an undercut trigger guard that is radiused and also for finger grooves before stippling.
found a post on the official glock page after searching countless pages.
I did it myself with a dremel, sandpaper and a 3/8" rubber fuel line. Probably could have went a little more, but depression with my big paws is much better so I stopped here.
I did this mod to all of my gen 3’s, I find it works much better than extended releases especially for guys with smaller hands. Best way to put it is the extended releases are easier to press, but this mod makes it easier to reach (and press).
I used a Dremel with 5/8" grinding bit to do most of the work and then followed up with some hand sanding. The key is simply finding just the right angle and holding it steady.