If you have the original openers/lifters, then measure the distance between the lifters where the new dome will attach with the lifters in the down position. Then take the lifters out by placing a screwdriver between the lower part of the lifter and the arm on the frame of the opening that juts out and pry it off. All of the work you need to do to the new astrodome, you can do on your workbench. Once completed, the dome can be installed from inside the trailer by lifting the completed dome through the vent opening and positioning it in place.
Hope this helps and good luck.
4CU 2699 / AIR 10 / TAC AZ-1
I'm haunted by aluminum.
Charter Member of the 4 Corners Unit.