I made a pin box mount for mine and bought a rack from Loading Ramps, Hauling, Transport, & Skateboard Ramp Superstore - Discount Ramps.com
Here are the pics. I did a write up on it as well.
Have less than $50 in mount and rack was around $149 I believe. With a Sidewinder, you make a 'clamp' with bolts to secure a mount around your flat piece instead of drilling through it. Then use a 'hanging' bike rack and use straps to secure them to the upright post of the rack so they will clear the rear of your truck. A few on here and rv.net have done them with short beds. Just a little ingenuity and you can do it.
2012 Sandpiper 365SAQ weighing @ 15k, Onan, 2nd air, slide toppers, TST
2013 F350 6.7L Lariat 4x4 CC DRW,Viair 1007, Ride-Rites, Edge CTS, B&W
2001 Chevy 3500CC DRW 4x4, 8.1L, 4.10 gears w/ Detroit TrueTrac
2001 Ford Excursion Limited 4x4 7.3L,V/B Spring Mod, ProComps, Hellwig Swaybar, & other 'Necessities'