I purchased a J&M radio system and needed places to mount the required antennas. Since my bike had a Givi trunk, I devised a pretty simple mount setup to work with it.
The mounts are made from aluminum stock 1/8" thick and 2" wide. They clamp under the Givi mounting plate. The plate is notched to position the mounts. Rubber strips bonded to the top of the two Givi brackets and an extra thickness of metal JB Welded to the underside of the Givi plate secure them when the plate mounting bolts are tightened. Since the mounts aren't actually bolted firmly in place, I feel that they could be deflected in the event of a crash, and possibly protect the rider. As a matter of fact, the mount was on the bike when I crashed in May, 2000, and it popped free (see my crash story).
The mounts position the antennas near the front corners of the Givi trunk. I've had them for several years and haven't had any problems with them.
The last two photos detail the rework to the Givi mounting plate.