You lost me at affordable. but then that is what cardboard boxes are for when they are flat. to fill up the hole the a/c doesn't fill. Don't forget the duct tape. Can't do real jobs like this without duct tape.
FYI the UO campus is using chill water at the rate of 3,500 tons of ice melting into water( the scientific formula used to measure cooling) per hour right now, and there is hardly anyone here! Well the lab experiments don't get to go home. They're always here.