I've had success with bungee cords. I also have an SUV with an adjustable roof rack. I use a flat 3'x5' piece of plywood with 2 2x4s set on edge very close to each other and bolted to the plywood base. I then slipped a thin fin between the two 2x4s and put bolts through them to secure the fin to the whole piece. I then painted the 2x4s blue for water and the fin grey (of course). I've been able to go 55mph with no problems. Good luck!
Posted By phinns
Daron - If you're going to drive with the fin on top of your car, I would use a nylon rope to tie it down with. You can get like a 1/4" rope with a strength of about 500 pounds.
Posted By NorthernParrot
This half-time stuff may be really neat in person, but sure is "corny" over RM. Would prefer what was done in previous shows where S.H. would interview special guests or members of the band