At about half the price of starboard is another UV resistand HDPE plastic called Seaboard. I've bought it at J Freeman in Dorchester, MA. They were easy to deal with and the stuff seems the same as starboard that I've worked with. They will cut to size.
http://www.jfreeman.com/cpg_seaboard.html
Most glues and caulks won't stick to these products. If you need to prevent water from leaking in you might want to use plumber's putty to form a non-permanent gasket.