I recently got a Wahoo Elemnt Bolt to replace an old and quirky Garmin Edge 500, and discovered that although the mount looks like a Garmin mount, it's rotated 90 degrees. I had been using Bryce's Garmin stem mount and he was nice enough to rotate the mount for me, but the dimensions are also slightly different so the fit on the Wahoo was quite tight... so I wound up redesigning it. I had also seen a request for a Wahoo mount that is compatible with the Bontrager Blendr mount, so I used this basic mount design to work with both of the designs, steerer tube and Blendr mount. This one is the steerer-tube mount, it replaces a standard 1 1/8" headset spacer to mount the Wahoo over the stem.
I've also included one that replaces the stem cap and a spacer. It requires a flat head M6 screw in the range of 25-35 mm. The flat head will distribute stress through the cap more evenly, whereas a standard socket head would concentrate all the stress on one layer boundary and make the cap more likely to break. Warning! I printed one in PETG and when I used it on my bike, it didn't break, but the whole curved top cap surface just kinda sank down. I think staying with a conventional aluminum top cap is probably a better idea.
I've printed this out (with a standard aluminum top cap) and have been using it with great results on my trail bike.
AIO Robotics PETG
Supports are needed for the top of the Wahoo mount, although I have forgotten to enable them and have gotten some usable (if a little messy) prints.