Theo has figured out how to use it fairly quickly, although he still has to think about it each time. Nahum and Willow are still perplexed.
After only a few hours of use, a few potential problems have been noted. Willow likes to park in front of the door, blocking passage to the other cats. There also have been some difficulties due to one cat wanting to enter while another wants to exit. A possible solution is to build a second cat door right next to this one into the same window frame.
I did a little online search and ran across this one that looks really nice, although it would not solve the aforementioned possible problems.
Ideal Sash Window Pet Door Cat Flap White - Flap size 6-1/4
The $120 price tag of this nice aluminum version inspires me want to continue the development of the current prototype. I found the cat door part at Goodwill for $3.99 and the other materials are existing from the 'shop'. If I can find another cat door, I could easily make a duel door version.