Assuming you intend to hang this off the roof rocket launcher. If so, I think I'd want the bottom of the new rod holders to be no lower than the ones on the rocket launcher. Anything lower than that and I'd be watching for someone to end up with a rod holder imprint on their skull since it would be hanging over the edge. Not criticizing your idea, that's just what pops into my mind.
Maybe you could mount it so it faces forward instead of behind it? As long as the anchor light etc aren't in the way. And if it's custom work anyway, maybe they could just add holders to your existing set up, unless there's a specific reason you want it to be removable. If you also had a couple rod holders on the transom where this could be mounted and you could move this from there to the rocket launcher that would be interesting.
Antidotes fix with the clamp on holders also seems like a great solution.