Exterior seam molding source needed

DrewE, thanks for the information, I didn't want to get into fiberglassing it if I didn't have to, that's another expense and ton of time I don't have. I've coated the plywood with 3 coats of spar varnish for marine applications. I was thinking of caulking the joints and then painting the whole thing with a couple of coats of white paint. But then a friend suggested using a molding at the joint which he thought I'd find at an RV parts store, but I haven't found that particular molding (not sure but I thought I've seen RV's with such stuff on it but could be mistaken). Same for the corners, just trying to save time if I could find some kind of pre-made 3/4 round plastic or fiberglass parts. Any idea where I might find such an animal?