Updated: Jul 5, 2020
We decided to remove the windows that we don't need in places like the bunks and bathroom. By doing so we will get a much cleaner look on the exterior and better insulation. Some skoolie conversions I've seen will use sheet metal that is way to thin and they have trouble getting it smooth.
In looking for a better solution I purchased 3 dent and scratch sheets of Duraplate (google it). This is what I am using to replace the windows that I do not need. It is AMAZING stuff designed by Wabash National Trailer manufacture for the sides of semi trailers. It consists of a sheet of HDPE sandwiched between 2 sheets of treated and painted steel. I paid $32 for a 4x9 sheet of scratch and dent Duraplate.
Years ago I used sheets of this to extend the sides of a dump trailer I used for construction. After a decade of hard abuse there was no rust or any signs of it wearing out. It is so strong at only 1/4 inch that if laid flat and supported only on the ends you can stand on it with almost no deflection.
Replaced the first 3 windows today with Duraplate. It was really easy and when painted I think it will look great. By using the Duraplate I did not have to do any reinforcing and there will be no ripples.
After removing the windows I cleaned up what little caulk there was. We laid down a thick bed of Sikaflex 221 and set the panel in place. Held it in place with a few scraps of angle iron. Lastly used the Sikaflex to put a smooth bead of caulk around the outside to leave it ready for paint.