Build a ladder rack.
Overhang the cab in the front and about a foot or so out over the rear bumper (check local laws for how far a load can hang out the back before getting a ticket).
I built mine so the feet sit on the floor of the bed and use turn buckles to the tie down points so it can easily be installed and removed with no holes in the bed.
It is 12 feet long and 4 feet wide. built from 1.5 x 1.5 heavy wall galvanized square tube and flatbar.
We carry 16" - 18"
signs without issue (no freeways here) with my Toyota pick up.
When it is out of the truck it can be used as a table or work platform with some plywood laid on top.
It's not pretty (about 20 some years old) but the truck it sits in is even uglier....
wayne k
guam usa