I wish I had a pic from when this was under construction because I'm not sure the words will adequately describe.
A fiberglass guy made a recessed frame that was then fiberglassed into the window opening. The new frame allows the pane of glass (or plastic) to mount roughly flush with the outer walls of the boat. Another fiberglass bezel was mounted so the glass is sandwiched between two pieces of fiberglass. On the inside, a wood trim bezel was fabricated to cover the fiberglass frame (second pic, sorry for the blue tape - being varnished).
Pretty sure this is superior to the original welting, but labor intensive. However it's an option for the suitable motivated
I sort of like the approach from the archives where the guy put a cannon heater (propane) on the dashboard and slowly bent the plastic window into place. I'd screw it in, then use some sort of trim to hide the window edges, similar to the fiberglass trim on my boat. Would be strong and water tight.
1970 Willard 36 Sedan Hull #40