There are 3 mounting holes on each side, and the 3 holes on each side are 1.5″ apart from each other, so that gives 3″ from the two outer holes. I could not find a u-bolt that was 1/4″ so I used a larger u-bolt which caused me to have to slightly drill out the holes for the u-bolt to work.
https://www.amazon.com/Hopkins-20400-Engager-Battery-Monitor/dp/B0012ARRCQ