Oh...one downside (no pun intended) about the Davis gizmo vs. a ball & wire. I would rather troll at 7kts in order to cover more ground and find out where the game is lurking.
Davis recommends trolling at 2kts. I probably go at 5 and push the limit but it seems to work. 7 is too fast.
Given the forces that a conventional downrigger exerts, It would likely take an aluminum pedestal bolted to the deck with a mounting plate for the unit, sufficiently close to the transom to allow the arm to extend. Don't underestimate the force applied with a 10 pound lead ball at 7 kts or so. Another word of caution on strength; on more than a few occasions when the ball was retrieved, the lead was deeply scarred with tooth marks! Were the unit not attached correctly and a 50# gamefish to grab your ball, oops there goes the downrigger, ball and all.