I designed this after the Stark SE-5 Express. The Hole is meant for M5 screw that is 35mm in lenght and a Nut that 8mm in Side-to-Side Lenth. If the grip is too tight on your picatinny rail, then I recommend heating the print a little and then sliding it back and forth to your happy with resistance it gives.

The inside of the grip is somewhat hollow, as shown in one of the screenshots. I did this to save on print fillament. When slicing the file, you can also see how hollow the inside is.

hint: if you are not experiencing warping on the flat side of your print, then I recomment printing the grip at an angle, like on the pictures, or where the picatinny is facing perpendicular to the print bed.