So, you just ordered 200 boxes from USPS because they were free and you've probably used close to none. Heres a neat project I made on how to utilize the smaller boxes. Don't have any boxes? Go to USPS.com and order some, they're free.
Step 1: Get a Box, Razor, and Packing Tape
In this picture I have two boxes because I wasn't sure how much I needed, get two in case you need another. Then get a razor and/or scissors and packing tape from officemax, walgreens or where ever.
Step 2: Tape up the box
Next get the box so its fully folded up, as if you were about to ship something in it, but instead tape everything top and bottom and then the sides (See pictures for reference) This step makes it almost air-tight.
Step 3: Grab your vent
Get your vent out of its hole and close it and put it on the top or bottom, doesn't matter, then outline the part where the air comes through. I would put it more to one side if the vents on a wall.
Next, cut the outline you just made with your razor and pop out the cardboard.
Keep the cardboard that you just cut out, you will need it later.
If you wish put the vent in the hole to test if it fits... Mine does, perfect fit =)
Step 4: Put it over your vent and cut a hole
Simply put it over your vent and check if any air comes out the top or sides, it should a little at the bottom. Now mark with the razor or a pen which way you want the air to go in the vent, I want mine to come out the side pictured so I cut a little gash in the middle of it.
Next I got my razor and just cut a hole, didn't really madder on what size, but its pretty big on mine. Once again, keep this peice.