The comms are positioned 8 inches behind the wing spar to center of antenna. I did not put them closer due to interference with ailron push rod, electrical connectors and etc. I read the wing bottom itself will help with the ground plane and make for great reception. The top mounting doesn't have a large flat area, if you think about it. It is all curving away.
I mounted these doublers the same way and same size as the transponder antenna.
I had a tech advisor come over and he said just using a doubler like this is what is needed.