The concept here is a simple 3d-printed jig that aligns two holes at a specified angle. The holes are drilled using a masonry bit in a hammer drill.
Step 1: (not shown) Drill a shallow hole, maybe 1/2" deep. This will be the start of the first hole.
Step 2: Feed the drill bit through the jig and locate to the spot hole drilled in step 1. Drill the hole to the full depth of the drill bit.
Place a dowel through the jig into the first hole.
Place the drill bit through the opposing jig hole and drill
Drop a line with a strong magnet into one hole… wiggle it until it hits bottom
Fish the magnet line through using another magnet (here glued to a stiff rod)
Secure the anchor cord to the fish cord using a series of half hitches. Pull through.