Starboard..
Starboard or Seaboard would ideally be a great product BUT nothing likes to stick to it in terms of sealant, tape etc. for very long so it may leak again in the future. This is why I did not choose it for this particular application. I have lots of it but also had some Lexan too. There are many ways to do this but this one, using Lexan, seemed the easiest to me..
When I filled my holes, three of them, I chose 10mm Lexan (polycarbonate) which I then painted matte black with Krylon Fusion plastic paint. I filled one hole with a speaker, which I use for my VHF and the other two with the polycarb blanking plate and two ST-60 instruments. While not my "ideal" installation it looks decent, stays dry and does not stick into the cockpit too much, only an additional 10mm.
I was able to countersink the clamping screws and hide them under the new ST-60 instruments so they were entirely hidden. Don't really like face screws if I can avoid them. The Fusion has held up extremely well and has not yet chipped or scratched. Ideally I would have liked to have filled the holes with fiberglass & finish with gelcoat but this project took considerably less time.
I was going to re-seal it with butyl tape, but after the test fitting with the neoprene foam weather stripping tape, I never did. The weather stripping has been bone dry thus far so I'm in no rush to replace it.
Cut, shaped, painted and the mounting screws are countersunk into the face:
The back side with instruments installed and the foam weather strip that has kept the water out for over a year now (I did not paint the back side, no need):
Finished install: