Teardrops n Tiny Travel Trailers

i'm wrestling with window ideas lately. I'm getting really low on money and the last big investment i have to make is on windows and a vent. so If anyone has any ideas for cheap alternatives let me know!
here are some thoughts:

1. I'll have to sleep in the teardrop in big cities, so I need the window to be able to be open but also secure and private (metal grate?)

2. I don't think i can afford to pay $60+ for vinyl windows and i'm having a hard time finding something reasonable that is used. so maybe i should consider slapping up some plexiglass? sealing it?

I got my windows from a salvage yard. Really if you are looking for open but not allowing a hand inside then some that crank out. Keep looking!