Just to give another point of reference to consider, I fabricated two gussets for mounting my magnetometers on the upper longeron (sp) behind the second bulkhead back from the baggage bulkhead. One on either side. It works great in that location. I used Velcro and brass screws to secure the magnetometers to the gusset. If I can find a photo I will try to post it here.
Know that if you have a slider, that second bulkhead is as far forward as you should mount the magnetometers. If you mount further forward than that when the slider is open the steel frame of the canopy will cause disturbance in the magnetic field.
I found some pics of my magnetometer mounts that I took during construction. This pic shows the location of the two magnetometers with the gussets riveted to the current bulkhead location.
This pic is a good shot of how I mounted the gussets. However, you might notice it shows the gussets mounted to the baggage bulkhead instead of the second bulkhead back. This position had interference from the steel canopy frame so I had to move it back to the second bulkhead where it currently sits and does great.
P.S. These are GRT magnetometers. You did not state which manufacturer you are dealing with. The magnetometers may be of different design than mine so your setup might require a different mount than mine. At any rate, I wanted to give you a different perspective to consider.