Brad: What kind of boat are you putting it in? Your profile dosent list a boat?
When I swaped from a XMD-3 to a CMD-4 I didnt need to but I bought a pre made plastic trim peice that fixes the Exact problem your talking about. The peice was made for a Air Nautique and it was a quick and easy fix so that why I bought it.
You can make your own out of plastic. If you have a local plastic supplyer in your aera you can templete the hole from paper or cardboard you need to cover and then re make it in plastic.