Believe it or not, the air currents in the back of a pickup will actually hold the airplane in. Unfortunately, due to murphy's Law, and an odd wind gust here or there, it's a good idea to have SOME protection.

A simple rack made from PVE pipe and Foam Pipe Insulation would work well.

Lots of people use them, now if sombody will post a picture of one, I won't have to draw it .