Thanks, everyone for the tips.
Another follow up. I found a glass shop that cut the pane to my measurements. On our boat it's a very dark bronze tint called privacy glass as used in commercial vehicles. It turned out that while my length/width measurements were accurate, the original pane was very slightly trapezoid in shape. I should have taken the original pane out to use as a pattern, but didn't want to leave the open space protected only by plastic as there were high winds and rain in the area at the time. Anyway, the mistake cost me a second pane of glass...$68 times two. Lesson learned...don't bet on boat window frames being "square" even if they appear as such.
In the interim the boat was hauled and placed in the winter storage building. The second piece of sliding glass fits well, although I noticed the felt track liner is badly degraded. So the search is on for that material. Gluing the second pane will happen in the Spring as I ran out of time this year.
Again, thanks for the advice. I think I found some urethane in a small squeeze tube. Here are some photos of the area. The sliding pane is behind the screen. It slides behind the fixed glass that is just forward of the screen. That fixed pane obviously had to come off to get at the sliding glass...and it needs to be glued back on once the slider is installed.