The solution - the all mighty duct tape, a few thumb tacks, a bit of screen and a bit of ingenuity.
Here's how I did it:
• Measuring tape
• Straight edge (or metal ruler)
• Duct tape
• Thumb tacks
Step 1: Diagram
Above is a diagram my father-in-law made to better visualize this process. (Obrigado Moacyr!;-)
Step 2: Cut screen
Place the screen against the frame of your window, and then fold in half against the window. Mark your cut point with a piece of chalk or crayon.
When cutting your screen, you want to try to cut as straight as possible. This is where your straight edge or metal ruler comes in handy.