I don't like to depend on windows for ventilation. Windows can supplement what you have, but I really like having adequate passive ventilation, something that is always there and I don't have to open and close it on a regular basis. I will admt that during the summer, being able to open a window is nice. Hopefully you will have some overhang or some arrangement so the rain does not come in through an open window.
You might consider covering the space with hardware cloth to prevent predators from getting in. That is there permanently, 100% of the time. Then get a sheet of plexiglass and arrange to temporarily attach it. There are different ways to do this. I'd suggest putting the hardare cloth on the outside so you don't have to worry about a predator ripping the plexiglass off. I'll say plexiglass but use whatever hard clear or semiclear plastic you wish. A couple that I haven't seen mentioned are:
1. Set some bolts in the wall with the bolt sticking through to the inside. You can drill holes in the plexiglass and fit that over the permanently set bolts. Secure the plexiglass with washers and buttefly nuts. Pretty quick and easy to put on and take off.
2. The idea is to make it like a guillotine door. Whether it slides up and down or sideways would depend on how much space you have. I'm not sure how thick your plexiglass is. Say it's 1/4" thick. Take a strip of wood that is thicker, say 1/2" thick, and attach it to your walls, either top and bottom or on each side, depending on how your window will slide. Your plexiglass needs to fit inside these with enough room to slide. Then take wider strips of wood and install it over your first two strips, forming a groove your window can slide in. It may sound complicated but it is really not that hard to do.
If you really want to be simple, just permanently attach a sheet of plexiglass over the opening and rely on other ventilation. If you do this, I'd caulk around it to prevent water from leaking in behind it and rotting the wood if it is exposed to rain at all.
Many ways to do this. Good luck.