I put an old storm window and an old screen from one of the basement windows in the space where the side door of the garage used to be. *

Here I used an unneeded screen from one of the old double-hung windows on the house to cover the window opening on the other side of the garage. *

I even found what looked like an old granite window sill and a piece of cinderblock/concrete to put at the bottom of the door so my storm window won't rot right away! Yeah, I know, I need to replace the window pane. Another time. *

Ta-da! You can actually walk into the garage now! If I got rid of some more stuff I might even be able to put a car in here! Not that I would, but I could. *

After some more clean up I found an aluminum-framed window that used to be part of a triple-track setup but had no home. Luckily it's almost the right size for my garage window! A piece of scrap 2x4 over here... *

and one screw up here... *

and another down here and a) the window covers all but about a 1" gap at the top and b) I can use the original catches on the window's frame to hold it in place! Easy removal, too! AWESOME! *