An old shirt(bigger the better!)
First, Cut the sleeves off. This is going to be the holes for your arms, and depending on how "baggy" you want it, will depend on the cut. I wanted it fairly baggy, thus I cut as close to the sleeves as I could get. This is an XL shirt, so I didn't need to cut that much off. Next, cut off the collar(same as the sleeves), then turn it inside out.
place tape along the bottom of the shirt on one side. To seal the tape, rub over it a few times. Next peel the waxy part off, and you should have a very sticky line of tape. After this, take the other part of the bottom, line it up with the tape, and rub it to stick. To double secure it, place tape on both sides of the bottom.
All done! I LOVE In-n-Out. Do you think they'd give me a free burger with this bag?! Think of how many burgers I could fit in this bag...Worth a shot!!! :)