Al,
When I installed my replacement rudder bow in, I wanted to protect the paint job and burnished surface, so I used 1/4 inch styrofoam panels with a plastic bag sleeve open at the bottom. The plan was to pull the bag up once the rudder was in place. I did that and it worked.
However, one of the benefits I hadn't planned on was the styrofoam panels made the rudder neutrally buoyant so it was very easy to position and line up the rudder pins. I'd like to say as an engineer I planned it that way,.... but I really can't!