You can make a carrier out of 1/2 in. or 3/4 in. pvc and custom fit it to your plane, strap it down with rubber bands. Whether or not a plane will stay in the back of an open pickup bed depends on the aerodynamics of the cab and how the flow behind it goes, and it will vary with speed, and passing another vehicle on the 4-lane can disrupt the flow around your vehicle.

Last summer I was on the way to the field, had my plane in its pvc"cradle", but not tied down. I was going between 65-70, and pulled out to pass a semi. As I drew past him, and looked behind me to see if I was clear to move back over, I saw something move out of the corner of my eye, and glanced in my rearview mirror just in time to see my Rascal 40 do a flip (I had it in the truck bed with the tail against the front of the bed) and the fuse came down over the tailgate and broke the plane in two just in front of the empennage. The control rods were the only thing that kept the tail from leaving for good, and the plane was bouncing around like the whole thing was going to depart my truck for the underneath side of the transfer truck that was now just behind me. I managed to pull off the road without getting run over or losing my plane. The fuse was crunched, but I managed to repair it (rebuild it) and I've had many good flights since. Since then, I have never put a plane in the back of my truck without securing it to its cradle.

I think if you're not going far or very fast, you might get away with it. One of my friends comes out with his GP DR-I just sitting in the back of his pickup, not tied down or anything, but he says he never goes over 35-40, and only lives a couple of miles from the field. I think the "bow wave" of air from the semi disrupted the airstream around my truck enough that the turbulence grabbed the tail of my plane. I had carried it many times before without trouble, I was just lucky.