FWIW, fork tubes are 53mm just above the triple clamp. I was able to pick one up from STG since XT was out of stock. My forks have 1 line showing from the top and the holder fit like a glove.I designed one similar to what's provided with typical fork tube transponder mounts and 3D printed it to hold a Garmin GLO for my Panigale and Ninja 250's forks. In my design I have the Panigale's forks to be 53mm so it would fit better with my N250's 37mm forks, but 58mm sounds plausible for the area between the triple clamps if you wanted to match the OD completely.
Where are you finding fork tube lap timer holders?
Nice! I was fairly certain I measured it with a caliper but wasn't sure if the 53mm I had in my design factored in other things or exactly where I measured.FWIW, fork tubes are 53mm just above the triple clamp. I was able to pick one up from STG since XT was out of stock. My forks have 1 line showing from the top and the holder fit like a glove.