We are converting a Crown Coach. Crowns came with a single air door that went inward when opened. They also had a huge rubber strip opposite of the hinge. Good so that students' arms and legs did not get bruised or broken if the powerful air door closed on them. Not good for wind, noise, critters, etc. See photo below.
We took the old door skin off, and extended the frame so the door completely filled in the door opening. We used a 3.5 inch steel extension that was welded to the end of the old frame.
We then cut out the holes for the windows.
We put a deadbolt keyed lock (keyed both inside and outside) on, painted it, put new rubber and windows in, put new rubber around the door and jam to create a more air tight seal, and finally adjusted the air actuator system for the door mechanism.
Total cost for all of this was about $600. But we have a door that was redesigned to work with the air system in the bus and maintained the character of the original Crown door. It looks great and keeps out the weather, road noise, wind, bugs and critters. We created our own screen system that uses magnets around the door frame when we want the front door left open and bugs left out.