Antenna Specialists used to make a clamp mount. Was welded to a pair of vise grips. All you need is some flat plate, weld it to the top of the jaws in a manner that will fit where you want it to. Have it welded up. Drill a 3/4" hole for the mount for the Motorola style MOT mounts. A mount to hold up an 8 foot whip (more efficient at CB frequency) might require a different sized hole.
Get the mount first, standard fender mount, or side mount. Fit it to the plate and be happy.