Robert, from what you say this "window" was actually an opening made for the A/C, rather than an original operating window with a sash.
You can put in a single pane of glass, but if you want the insulating value of a double pane don't caulk two separate panes in; you'll just wind up with a moisture problem. Instead, check with a local glass company about having a thermopane unit made up for you.
Yes, this is in effect a "custom" piece of glass, but it might surprise you just how inexpensive this is. The glazing company doesn't really see this as custom because they cut glass all the time to fit standard size windows, too. The cost is based totally on the square feet of glass needed, not the cutting, unless it involves curved cuts.
Have the unit made to fit, and you can then trim in the thermopane unit yourself.