Easy way: make a new back glass out of clear acetate (you can find it at art supply stores or Hobby lobby). Get a piece of thick, clear acetate larger than the window, cover both sides with masking tape to protect from scratches as you work. Lay the kit glass on the acetate, trace around the kit glass, then trim your new acetate "glass" to size. Measure and lay out a center line on your new glass, draw the shape of the duct opening, and use a sharp X-acto to cut out the hole. Clean up any rough spots with a fine sanding stick. Remove the masking tape, add the duct body to the underside (using Jairus' references above), and there ya go.
If you mess up the hole, no problem. Try again with a new piece of acetate.