This past summer, I installed new fixed portlights in my 88 C30. In the originals, the lens was held in place with screws around the perimeter but I used 3M VHB tape and Dow 795. Here are some photos. The three vertical strips of white tape form a hinge to allow the lens to be properly aligned and then rotated up out of the way to strip the paper off the back and wipe down with alcohol before sticking to the tape.
I removed the old lenses with a kitchen pairing knife that was thin and flexible enough to fit into the small gap and cut the old silicone.
I then used the original lenses as a template to cut new "dark smoke" cast plexiglass to shape. I ordered the rough shapes from Tap Plastic online. On my C30 the lens material was 1/4" thick but that should be verified for the C310. I was able to find a location on one of the lenses where the factory sealant did not come the the face and was able to measure the thickness there. You could also pull one of the screws to check the thickness.
After drawing the shape on the protective paper on the sheet of plexiglass, I cut the rough shape on my bandsaw and then stuck the old to the new with SailRite SeamStick tape.
Then I used a flush cutting router bit to trim the rough blanks to the same size as the originals. After switching to a 45º bevel bit with bottom bearing I cut the 45º bevel with bearing into the back side.
The original lenses were tight fitting with almost no gap around the edges with a very thin layer of silicone between them and the cabin. If the sealant is too thin, it can cause leaks from thermal stresses so i needed to make the new lens about 1/8" smaller. This also gave me a big enough gap to gun the silicone into. I did this by adjusting the depth of the 45º bevel deeper to cut the extra 1/8".
Next was cutting all of the old silicone off with a sharp chisel and cleaning it with sealant remover and sanding. This was the most difficult and time consuming part of the installs. We then applied VHB tape to the edge of the opening and used a razor blade to trim any that extended past the edge of the opening. We then positioned the new lens in the opening and when happy with the location, added three tape "hinges" to the top to hold it in position. With the lens rough fitted in place we attached masking tape to the perimeter to form the caulk joint line.
The lens was flipped up, paper removed from back and the inside face of the lens was cleaned with alcohol. Then the backing on the VHB was removed and the lens rotated back into place using the tape hinges to hold it in the correct place. The alignment was perfect.
The project was then finished with black Dow 795. Note that this adhesive takes a very long time to cure so do not plan to use the boat for half a week to a full week after install to allow the 795 to fully cure. The benifit if the long curing time is that it does not tend to "skin over" while you are applying it so it is very easy to finger the joint after you have gunned it into place. While gunning the caulk, be careful to get the gap completely filled. I used a pushing technique rather than pulling because it gave me better control of how deep it flowed into the joint.
So nice to have clear window for the first time in 20 years.