The Holosun 407K is a direct mount on my Tisas Nightstalker Double Stack, but I found that even with the elevation adjustment to the limit it was still shooting too low. After getting a shim off of Amazon only to find it wouldn't fit on the pistol, I remembered I had four 3D printers and could probably make my own. Happily, my first try worked great.
I printed this on my Bambu P1S with a 0.08 High Quality profile. I wanted as short of layers as possible as the thickness only drops from 1.25mm to 0.75mm. I printed it in ABS but it should work with your filament of choice.
The author marked this model as their own original creation.