If you want to put a hole in a bottle cap and don't have a bit large enough, but have a Dremel with a 561 Multipurpose bit, this offset jig will make it easier to get a (mostly) round hole.

Fusion 360 file sketch dimensions can be changed to adjust for bottle cap diameter and desired hole diameter.

FYI: the Dremel collar is a 3/4" 12 TPI UN thread.

Update: the F3D file is slightly improved to handle parameterization better.

Instructions

Start the bit at an angle to the top to make an access hole without the jig in place. Screw jig onto Dremel collar, place bit in the hole, then using the outer ring as a guide, slowly rotate the cap around. It helps to use both hands.