EDIT: I cant post links, so the ones below you will have to modify a bit when you paste them in your browser.
If you want 'real" concession windows, you could start with some RV windows:
triple dub -dot- pellandent.com/RV_Products_List
. -dot- spx?CategoryID=25
There are a few models in there that would allow a narrow passthrough at the bottom of the window while keeping the top closed.
Better yet, this company makes them ready to go:
triple dub -dot-buysellfind
post edited by ductape - 2013/01/17 14:31:47