I really like @Adam's excellent tri-glide slide model. It's simple and prints quickly. I've widened the threaded version to fit a 1.5" (38mm) strap.
UPDATE: @Adam has added a 1.5" width in both the threaded and non-threaded versions. I recommend using it instead of my remix.
The author remixed this model.
Widened to fit a 1.5" (38mm) strap.