Somewhat related, I tried a canvas repair kit, which was mostly glue and dye to color the glue, to fix a small tear. The spot now looks like a dark wet spot on my tan top because the way the glue dried, and it looks like ass. I don't anything about this marine goop, but test it out before you use it, and consider tapping off any areas on the exposed tarpaulin.
Our cars have a separate rear window section, you might be able to get that section for a reasonable price at a junk yard. It won't be a self service yard, only smaller places hold onto wrecked converts.