Don't cut the corner. Roll the corner doubling the normal roll hem width you use. That way when you roll the edges it will give your corners a nice finished edge and a hole to run the cord out and around the perimeter. Just my thoughts, good luck.
Yosemite Sam: Are you trying to make me look a fool?
Bugs: You don't need me to make you look like a fool.
Yosemite Sam: Yer deerrrnnn right I don't!