The new MC rudder port I used was also bigger on the OD of the tube. I was originally planning to bolt plywood to the hull, mark center, and drill appropriate size with a hole saw. Turns out it was not really much bigger. I used a small 3 stone cylinder hone on a drill with 40 or 80 grit sandpaper wrapped around it. Didn't take too long to open up the hole and I was able to sneak up on it for a snug fit.