One thing I would say is you don't have to go down both sides of the board. You only need a slot on the inboard side. There's no need for 2:1 purchase to get the boards up.

I guess use a router and then glass it back up to seal it - assuming there's foam inside. You may want to make the slot a lot wider than the rope though to help avoid it jamming in the slot.
On teh other hand I guess you could also cut out a complete hole right through - use one of those 50mm holesaws at the top and bottom. That could work - and no fear of the rope jamming then.