Have 2 12s mounted similar to what you are showing. The one on the side is mounted with a ballzout mount.
I'll second looking into BalZout. They're not cheap, but they also will never move once you lock down your knobs. RAM swing arms also work to save some money. The Helix 9 is just about too big for the RAM universal clamp-style mounts to hold steady in rough water consistently.
I thought about a BalZout and not sure if the place to mount it , on the rod tip pad , if the glass is stought enough there to absorb the shock of rough water. That Helix is pretty heavy. Anyone with a Skeeter know if I'm doomed for a cracked top cap? I could put a doubler under it. I haven't looked to the access to do that. I bet its doable.