The first thing that came to my mind is to glue first side of velcro around the window and sew the second side to the mesh, so that there will always be possibility to get rid of the mesh. Another option is to use zipper instead of velcro, but as far as i understand, that thing will require much higher sewing skill level from you to make it look nice.
Actually, i’m not even a newbie here - just came to the website and this is pretty much the first question i saw here, but i used to think of all these van/bus things hard for the last couple of months. And that is one of those questions i asked myself.
So maybe there’re some already made options on the market designed specifically for the model of the car e.t.c.