This is what I fab'd for a neighbor's kid.
I attatched a hanger(bolted) to each frame rail, one using round stock and one side using square,
One side of the cross bar has a square section of tube that slides into the quare hanger to prevent the flaps from twisting in the hangers. The cross bar then slides through the two hangers, and I use a hitch pin to hold the other mudflap/mount onto the bar. It breaks down and the flaps roll up so it would easily fit into the truck tool box he has for when on the trail. IIRC, it cost less than $15 in supplies.
Here in CA you are only required to have one not both mudflaps and flares, so he didn't need flares as well to get his fix-it ticket cleared.
If you have clearanced the rear bed sides, and have room to come off the frame, you might be able to do something similar to what I did for my '91 flatbed.
The mudflap hanger just slides out, but you might be able to reverse the hanger(slide it in from the bottom) and use a hitch pin to hold the hanger in place, or a castle nut and pin for extra security.
