Hi Shauna, You've got an odd duck! Odd ducks are fun and not unheard of in the Airstream world.
Could you do us the favor of posting the serial number from the plate next to the door? That will clear up what model year you have (as opposed to a registration year).
Ed is right that 1966
trailers all (or all that I have ever seen) have Phillips-Corning "frameless" windows on the back. So 1966
It appears to me you have 1965
taillights. Those taillights were not used on any 1964
or 1966 trailers that I have ever seen.
If you have jalousie windows all around your trailer, you are likely to have a 1964
model year because they did build a bunch of trailers in 1964
with tons of jalousie windows. I don't believe I have seen that in 1963
Regardless of the wonderful, unique, puzzle of a trailer you have there, your question was how to install a good looking canopy window where your jalousie window it now. The way to do that would be to find a salvaged Airstream...or other trailer. There are no new RV windows made with square corners...all are radiused. That wasn't true a few years ago, but it is true today. To do an efficient salvage search, start by having excellent measurements of your rear window and get a window that is that size or bigger. Bigger windows can usually be cut down (with some effort). Airstreams tended to use some of the biggest rear and front windows, so you may be stuck looking for a salvaged Airstream. Do a quick eBay search or Craigslist search to get you started. Also, post a "Parts Wanted" request here on these classifieds and also at Tin Can Tourists.
Best of luck!