To fill the corners you could order a gravel shield from Scamp
. I think these are front window cutouts anyway. Scamp
might even sell you some cutouts for less than the gravel shield kit. You can then glass in the whole window in and re cut the opening for your desired window.
Lay the cutout against your opening and mark the exact opening size on the cut out. Then cut the panel to fit the opening. Next pop rivet metal flanges covered with wax paper or a release agent to your original window pop rivet holes inside the trailer. Then pop rivet the panel to the metal flanges to fill the opening. After the panel is in place take a grinder and feather both edges and glass the seams on the outside the trailer. Then drill out the rivets and remove the metal plates and repeat the same glass procedure on the inside the trailer. This time glass over and fill the rivet holes on the inside. Then go back to the outside sand smooth and reapply glass again, this time covering the rivet holes from the outside. Then sand the outside smooth again and paint
. Then cut out for the new window.
I would suggest you try to find a slightly larger window to over lap the square corner with the radius of the new window this will save a lot of mess, time and work.