I'm trying to replace the windows on my 1976 Columbia. The windows are mounted from the inside and unfortunately the surrounding wood is rotted and needs to be tossed. The top of the window is flush with the wall but the rest is a variable depth with an inset of about 1 inch at the bottom. I need to bring up the bottom of the windows to be flush with the wall in order to properly bed the window. The material would need to be marine grade and screwable. Any suggestions?