I found a couple of 2x3s in the basement. Cut 2 pcs so they fit across the bed, one up against the cab and one about 21" back. Then cut some 24" pcs and laid them front to back, on top of the first two. Spaced them on 8.5" centers, and screwed them down.
This rack holds both standard and 'fat boy' tanks very nicely, though if you have fat tanks with boots, the tanks will be knocking boots. I've heard that's where pony tanks come from, but it hasn't happened to me yet. ;-) I only put one drywall screw (not very strong) at each joint, and it has lasted 10 years so far.
I've stacked three layers of tanks on this rack, and it works great. I use straps to hold tanks against the cab if I have more than one layer, but with only one layer I don't use straps any more. The tanks never move.
I might point out that the construction of my bed ('99 Ford Ranger with Duraliner) is such that once the rack is in, it can't move.
D
Oh, and it cost me nothing to build.