$20 WalMart Solution
I picked up a 'cargo net' from WalMart - really a bunch of bungee cords tied together. I drive ~35 miles to our field (65+ MPH) and with just that laying on the planes, they stay put.
Most of my wings get wrapped in a blanket so the aileron servos don't mess up the other wings.
Driv'n a '97 Dodge Ram 1500 (short bed).
Jerry