Found it. I was doing a walk through at Home Depot again for the third time. A couple of clerks asked if they could help and I declined saying even I didn't know what I was looking for. I looked at the plastic decking but it was too rough, didn't like the color, I'd have to do a lot of prep on it and it would never look right. I then wandered around in lumber looking at trim. Some stuff looked like it might work but it was compressed wood with a vinyl coating. I finally got way in the back and found the plastic molding section. It looks just like wood molding but it's plastic and cuts similar to wood. They claim it can be used in areas where moisture is a problem. Sounds like a boat to me. I finally settled on a piece that was a simple right angle, about an inch on each side and maybe 3/8 of an inch thick. It should trim out the opening nicely. I ran a test piece through the saw and it cuts nicely.