When I mounted the comm antenna last week, I used a piece of .060 stock as a doubler under the antenna base, with about 2" extending aft. That still oil-canned the upper skin, so I added an 8" piece of L-angle. That *still* resulted in oil canning and what I thought was a large amount of flex in the anteanna base where it mounts to the skin.
So I removed it tonight (was out of town for the past 3 days hence no progress...), and used a singe long piece of U-channel stock left over from my Sonex project. Match-hole drilled from the old doubler to the new stock, then added the old L-angle and match drilled that.