Downriggers on a 2010 Lund 1650 Renegade

I just bought a 2010 Lund 1650 Renegade and i love it! My problem is that i Salmon fish and i need to mount my Cannon downriggers which have a 4" mounting plate and the boat only has a 3 1/2" gunwale.

Does anyone know if there is an adapter plate etc. where you won't have to drill holes all over?