I replaced the rear/side windows in my CT with horse trailer windows earlier this year. Very economical, but took some fussing both to install, and later to chase down a few minor leaks
I described it on the last page of my build thread:
Under $250 for the pair, including shipping from Oregon(?) to Iowa.
Not a "high end" solution, but I'm quite pleased with them.
I had to trim the height of the opening about 1/4" top & bottom, and take about that much off the aluminum flange of the window itself. Because of the modest overlap onto the CT skin, I set them with automotive windshield urethane.
So.........not for the faint of heart, but an option............