Re:Concession Trailer Windows
In California the max opening is 216sq inches (and yes, it is supposed to have some type of closure device), so most trucks don't use standard windows. They just frame in a piece of plexi with a cutout for the food pass through. Often there is a large center piece with 2 sides with cut outs.
Since it's common and standard, it doesn't look strange to any one, and it saves a bundle off buying a framed window. Insulation is a non issue in my opinion, so nice plastic framed windows from Home depot are a waste of $ here.
I've seen examples of everything from heavy stainless framing to simple aluminum C channel holding the plexy. Unless you look really close, you can't tell the difference.
I think the closure device can be a screen or solid piece. A lot of trucks put a slide up screen on the inside with small aluminum C channel that just covers the cutout. Some hinge another piece of plexi that flips up and just screws into the main plexy.
If you're on a budget, this can save a bunch off buying a professional window.