go to checker or autozone or whatever, and get a set of van ladder racks. either cut of or leave the bar that holds them. something like 54.99 for a pair (four feet)
I have made my own before, with .125 plate, (4"x4") , a piece of .75 rect tube (2.5" long) some all thread and some .5" angle.
you center the tube on the plate and weld in place, and weld the angle to the allthread. Put the clamp (allthread) through the tube, and place a nut and washer on the allthread.
viola. done
or go to checker for less money and time