Thanks The plastic was just so my daughter wouldn't cut her fingers/hand on the cut edge of the window opening, she was using the window to give us pretend ice cream cones from her "ice cream store". Just a scrap piece of sheet plastic for temporary use.
We are converting a metal shed - I have an image on my page.
We bought a jigsaw for the project and it works great. We drilled holes to start the lines of cuts, and also used tin snips on some parts.
I found metal venting covers/squares that we screwed in over our back vents, I think they may be...