if the frame of the window glass is missing but the frame in the trailer has not been damaged I would consider making a plexiglass window and mounting hinges directly through the plexi. You should be able to find a auto window glass gasket in any restoration shop that will go on the frame and seal to the plexi.
I had to replace my glass with plexi after I put the handle bars of my bike through the glass. Jacking the frame apart to put the plexi in was a job. I had to make a jacking jig to do that.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles