You are right to suspect the battery cable. In my case, it was the line from the battery to the iFly that was the culprit. I was able to get a different cable from AP that has a magnetic choke on it, and that knocked the interference way down, but didn't completely eliminate it. I still have to be somewhat careful about how I route that cable, or I'll still get interference.
I think such clip-on magnetic chokes can be found at hobby electronics shops, or online. They're inexpensive, and maybe worth a shot before soldering caps behind the panel somewhere.