I agonized for a long time over where to put the magnetometers. I originally had planned the wingtip, but it is getting busy out there. Also, the recommendation is to keep them on the same line. I was looking at mounting along the side of the baggage bulkhead but couldn't figure out a good solution.
Last night, I decided that a simple angle bracket assembly would work well. I made the tray from 032 and bent the flanges up to give it some rigidity. The cross supports are designed to be about 8" behind the bulkhead. This allows it to be inserted up near the bulkhead, then slid aft. It is retained with four #8 screws.
The result seems very stable and out of the way. It is well away from any ferrous metals.