Question about making square holes in box car sides

[Rscott417]

Im trying to recreate ECOX box cars which were used to transport baled garbage. On the sides of the box cars they made 2 square holes for vents. I want to use these vents...<!-- m --><a class="postlink" href="http://www.walthers.com/exec/productinfo/173-353">http://www.walthers.com/exec/productinfo/173-353</a><!-- m --> and cut them to size. The box cars im using are the Walthers 60' Double-Door Auto Parts Box Car. My question is how would i go about making even square holes in the side of the box car to insert the brass vents?? This is what the cars look like and the vents are in the upper sides of the car.