not to clever with boat windows but i know what i think you need to do
ask a carwindscreen fitter lol
put the glass inside the rubber seal then get a long piece of rope [ cord washing line is the best not plastic ] put the rope all around the inside of the rubber seal [ not in the same place were you put the glass if you know what i mean ] then place the whole lot in the frame ..now the tricky bit
get 1 person to press the glass while you pull at the string all the way around [ dont press to hard tho ] and it should hitch into place pretty easily if it feels too tight pour some washing up liquid around the seal so it slides in easier,,,hope that helps craig
tellus how you get on or if you get stuck