Everybody has probably seen this one but me, however, I thought the Hand-e Hauler was the best solution to transporting LP tanks that I have come across. I was about to build my own solution when I came across this quite by accident at PEP BOYS.
http://www.northerntool.com/shop/too...0Cans-_-179061
Cost: About $10, bungee cord not included. Also hauls certain sized gasoline cans, and #30 LP tanks.