stick to the kit design
The attachment that is designed in the kit is designed to distribute the harness loads into the structure. This has to be done fairly carefully.
Attaching to the bulkhead, without carefully engineering the right brackets/braces is not going to be strong enough. Crash loads would just tear the bulkhead right out of the fuselage.
By the time you engineer something that is adequate, you won't have a clean look on the rear deck anymore.
Also, You can't ever see that area, it is always under the canopy skirt. It is just not worth the trouble to try to make it look any better. I have my GPS antenna and XM weather antenna in that area, and frankly, I never notice them.
so, its not like you are going to catch the sun on the rear deck and see the rivet lines - you just will never notice them.