I am going to buy some of the heavy mil plastic exterior curtains. Saw lots of them in northern michigan recently. These will be hung on my porch and to block my rear door.
And, we are working on replacing some of our interior curtains with over-sized, light-blocking, curtains.
And I use various bits of stuff to cover holes, including tape.
It sounds like you don't ever open these windows. If it were me, I'd make sure they were locked tight, then just caulk the hell out of them, preferably from the outside.
I don't open that one. I have 6 windows and I only open 2. Once the other 4 get sealed up, they'll stay that way. It'll also help keep hot air out in the summer. The spider tracks on the inside of the plastic just looked cool and it took me a bit to figure out that's what it was.
If I could caulk the outside I most definitely would. I'm on the second floor though so I'll just have to do what I can from inside.
In one particularly horrid Apt. that I had, I used bubble wrap. Cut to size, spritz the window with water and place the bubble wrap on it. You could use a broom to push it up. Worked best of anything I've ever used.