Pull out the center rubber "spreader" from the window seal...push the window out from the inside of the trailer(be sure to have someone outside to catch it....take the old window down to an industrial plastics dealer in your area...get them to cut one to size for you, using the old one as the template.....now take the new window and the old window seal all cleaned up, to a windshield repair store along with the trailer of course.
Ask them to install it for you and tell them you want a watertight seal (they will put extra goop on)
The curves you see in the window are automatically formed when the window is put into place.
In Canada, "Speedy Auto Glass" does good work
...This is from the 'other' forum...