I talked to my family to get their input on using U-bolts. They tell me that it will ruin the cardboard and be a terrible idea. My wife and daughter suggested I get a yoga mat strap. I looked online and found a strap for under $1 with free shipping. There were some fancy ones with adjustable straps, but they cost more and I'm a cheap guy. I figure I can find some way to tighten the straps on it.
I saw Yoga Bags for very cheap, but I wasn't sure if they would be long enough. Also, I like the FFG box and want to keep it in that.