I know that I need to order one. I have come down to the choice of either Perma-Cool or Flex-A-Lite. I like the cost of the Perma-Cool units. The question is what is a good size to go with, like how big is too big (I know you cant have too much cooling). Is the fan really necessary, I would like to have one since that would help when in traffic. Thanks for any input you guys can provide!