I've successfully carried two 2.5 gallon plastic gas cans on the factory bumper of my '99 Ford Super Duty for years. One one each side, mounted as far to the end of the bumper as they will fit and secured with cam lock straps. Unfortunately, I don't have any photos, but it has worked well. The bottom of the two cans I have are the same width as the bumper and ride there securely. The only down side is that the door of the Grandby won't make it to the door catch when I want to leave it open. In that situation, I use a strap or cord to hold the door open.
Not sure if this would work on non Ford bumpers. It does limit how much extra gas one can carry and doesn't look quite as elegant as a purpose designed carrier. But, it's cheap and does the job. I figure the 5 gallons could give me at least another 50 miles if I was in 4wd, more if cruising on the pavement.
At times, I have also mounted a Yakima basket case on the very rear of the roof where it is easily accessed. I have carried gas and other misc up there as well. However, it adds to roof weight and requires two people to pop the top up. Now days, I only use the basket case option if I am going to stay put somewhere and can unload it at the campsite for the duration. For road trips, I use the bumper option outlined above.
TK
