I was going up a ladder and needed both hands free and, necessity being the mother of invention, I took some clothesline, made a small bowline to hang the hammer then made a larger bowline to hang around my neck.
And it worked. My hammer was hanging right in front of me and I could get at it easily while standing on the ladder and holding a length of siding.
That evening I've got The Ashley Book of Knots in front of me and I'm thinking about making a paracord hammer loop for my belt. The basic design would be two loops, one to go around my belt and one for the hammer but I wanted to find a specific kind of loop for the hammer. I needed it to be open when there was no hammer in it, more like a circle.
I found what I was looking for in Chapter 11: Single-Loop Knots. Here's ABOK 1063:
From the diagram it looked like just what I wanted - and it was.
I cut off a 4' piece of paracord, made my loop in the center, brought the ends around and fed them through the knot, tied some square knots halfway up (to hold it together), then ran the ends down through the inside of the square knots and cut and melted the ends.
And my paracord, hammer loop has a significant advantage over the leather one. The leather loop is made to be stiff (to hold it open) but it remains stiff when the hammer is in there, so when you bend over, the hammer tends to slide and bending over far enough, the hammer falls out of the loop.
With my paracord, hammer loop, the loop stays open with no hammer - and with a hammer and bent over, the hammer handle (like a gyroscope) stays pointed to the ground and the loop twists and holds the hammer.
And I couldn't have made it the way I did without my Ashley Book of Knots.