probably irrelevant now that the OP has gone for another option but....
I have a Garmin 90 and when fitting it I realised i had to cut the 30mm hole. Really didn't want to cut the cable, and my solution seemed less hassle in the long run.
What I did though was to mark the ideal cable location, then drill a hole the right size for the cables. ie something about 6/8mm. Then I cut out the 30mm hole with a hole saw making sure that the outer diameter of the hole just overlapped with my original 'cable hole'. (sorry - this sounds complicated already!
KEEP THE CUT-OUT! I then put the cable plug through the 30mm hole and popped the 30mm cut out back in place with some 'marine sealant' and also sealed around the cable and the centre hole left by the hole saw. It's not the prettiest solution, but at least it keeps the water out of the console.
I've just used this approach again when fitting the new control box. All the connectors for the loom needed a much bigger hole to fit through than the cables themselves and I really didn't want dollops of water getting in there to the battery/loom etc etc.