Mine are through bolted to the outer ends of the bumper, which puts them about even with the inside of the tires
. Looking a Bolers in the Gallery, I see that the bumper mounts are closer to the center line of the trailer. This would be the easier way to do it, but not as stable. The only other option would be to put them on the rear of the frame itself. You might look under there to see if there is access. I'm not familiar with the way the Boler is mounted on the frame. Hopefully, who has used these on a Boler will be able to share their experience and/or method of mounting.