Well, I'm not happy with that, and I want to make sure the base of the antenna is really contacting the airframe more solidly. What I came up with, and I'm sure this isn't anything novel, is to use aluminum tape. Basically, here's the shape of the piece that I cut out (this is the backing strip after I already applied the tape). I folded the tape onto itself where those little flaps are. Those go between the nutplates.
The idea is that on the wing tip flange, the tape is sitting along the flange and contacts the wing skin across a large area.
On the inside, the tape folds over and contacts the antenna base. I used separate strips of aluminum tape to hold the folded-over strips solidly onto the antenna. Hard to see here. The key is to get the "outside" surface of the tape folded over and contacting the antenna. The adhesive side is not conductive (at least mine isn't).
Here's the underside of the top wing skin where the tape on the wing tip flange will contact.
See what I'm saying? Once the plane is painted, the screws will barely make contact with exposed metal, if at all. I'll specifically make sure the underside of this wing skin is masked off in this area to maintain contact with the tape.
Why did this come up at this point? Well, let me just say that I want to make sure that if I'm doing any radio interference testing, I want to rule out the possibility of poor antenna grounding. I'll leave it at that for now...and I'll say more when I have the authority to do so... 8^)