I put in a marine porthole in my door. It's one that opens with the port folding up. And yes, the screen door does get in the way a little. You have to open the door and separate the screen door from the solid door to open it, but once opened it stays up and you can close both doors. The other thing it does is that the port securing nuts (plastic handles on top of metal nuts) press a little against the screen deflecting it about 1/4 to 3/8th of an inch when the port and doors are all shut.
I got the porthole on Amazon. https://www.amazon.com/gp/product/B01CJK3VKE/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1
It cost $37.
The port has a 1 1/2" flange so you need to make sure your door is thick enough for it. I checked this before I bought it by taking off the lock set and measuring the actual thickness of the door. I barely had enough, but when you set the porthole, you can compress the foam in the door a little just by snugging down the screws around the porthole. You need to be careful sealing it with caulk since the flange connects to the porthole on the inside. On the outside there is a ring that fits around the slightly extruding flange. We've had a record rainfall here since I put it in with very windy storms and I've had no hints of leakage.
I'm thinking of adding a couple more, one on each side, up front of our 172 since it has no windows forward. We took out most of the top bunk for head room and it would add quite a bit of light and improve the ventilation. But that's for the future since now it's time to go camping.
Of course you can get a conventional rectangular RV window that will be easier to install and will slide open, but the porthole looks really cool especially on the retro form of the pod.
If you'd like pictures, pm me and we'll figure out how to send them. I don't have a public website where I keep fotos so I can't put them in here.