I made a vent window for my passanger side door(similar situation, found hard doors, passanger side was missing all the glass). The frame for the vent glass was gone as well, all that was in the door was the seal for the vent window. I was able to cut out a piece of lexan, and fit it between the two "flaps" of the seal. It's not ideal, as I can no longer open that vent, but it's sealed up well enough that it's kept the rain out for the past 5 years.
Here's the best pic I could find, you can see that there is a little bit of hazing, but for a cheep fix, it worked out prety well.