Well, now we can actually see the confines of your boat, it puts a different light on the subject somewhat.
personally i would prefver to "ditch" the bobs board and retrofit an electronic "esc", this saving you weight, a second servo and space.
Or
if you are reluctant to replace the esc, then mount the rudder servo as close as possoble, but without interrupting the bobs board operation, as close to teh bobs board you can. then the servo rod can run down the one side, direct to the rudder post, then it leaves the other side of the psace free to mount the second servo for the bobs board arm