Designed to hold a 1/2" OD x 1+1/2" aluminum bushing (I use https://www.thingiverse.com/thing:6124290) perpendicular to a surface to be drilled into. 1/2"-spaced holes for #6 screws allow it to be bolted directly to the workpiece, or to another jig piece.

The OpenSCAD design has parameters for inner and outer diameter, so can be customized to other bushings or for use without a bushing.