We are also looking at ways to renew the companionway, and the plexiglass idea is one we like.
I am no expert, but I have read that glued joints in plexiglass are nearly invisible. My thought was to remove the existing companionway by cutting horizontally, reuse the top rail, lock and hardware, and make the rest from plexiglass matching the thickness. Have the fabrication shop put a half-lap scarf joint (as low as possible) to field glue in place so that one does not have to remove dodger.
I had to look up the "half-lap scarf joint" terminology (google it, since I can't draw the shape here) so forgive me if that's not accurate. This is a joint that looks like a step on each piece, they fit together and give a lot more area for the glue to adhere.
I am considering sanding the exterior surface with 1000 grit sandpaper to make it opaque, and to render further scratching from the bottom rubber seal not an issue.
Any thoughts from plexiglass experts out there?