Back of an Open PU bed is generally a bad idea from my experience.. few years back I had three 50"~80" WS RC planes and no cover /topper/Cap and I did have a pretty good system to hold each plane down
Problem one was-- arrived at site to find a wing shattered by branch that fell out of a tree I had parked under at the gas station I stopped in for coffee
Problem two was--- a distraction during load out to leave... forgot to secure one of the planes...1/2 mile later, at 40Mph, it flew itself out of my bed
The solution, an enclosed trailer, was significantly cheaper than a good Topper for the F350
With a bed cover or good Cap/Topper---- Bob Z's method is very good, and can be made in just about any size that fits the space....I copied it for my in hanger use, made three 5 plane versions just as shown
PS do not use 1/2" pipe and 3/4" is iffy for bigger planes
When I did the project--- I had a lot of 1/2 pipe only good for my 1/2a planes as much too flexible for real airplanes
next try was 3/4 pipe cuz all the joiners were cheaper than the 1 inch parts... more than 4 planes is too much
Bit the bullet and build a 5 lane 1 inch pipe version....solid...nirvana!