Not much. Some 2x4's, wood glue (optional, but I had some so used it), 2 1/2 inch wood screws, 2 screw eyes and 2 ratchet cargo straps.
I cut the 2x4's to fit across the bed top & bottom, then 2 uprights. I pre-drilled all screw holes so nothing would move when I tightened it up.
The uprights are on top of the cross bars, they're the outer edges to set tanks against.
The small bits are also 2x4. I laid the assembly down, put a tank against the left upright & positioned the upper & lower blocks to its right. Marked position with a pencil. Removed tank, drilled screw holes (2 each), applied glue, screwed down. Move the tank & repeat across the gizmo. It'll hold 7 tanks across, no reason I couldn't have more than one layer of tanks.
I made the upper blocks 1 5/8 wide, lower are 1 1/2. My tanks have boots so I made the bottom 1/8 smaller to allow for them.
The uprights are 19, cross are 61.
My bed liner makes the top of the bed farther in than the bottom. I needed spaces on the top to use up the space & make it sit vertical, so on the back of the top cross bar I stacked 2 short 2x4's, about 6 inches. This is one 2x4 on the back, then another mounted on top of that one. I did this near each end.
Along the top bar, on the back side away from the tank, I also put a screw between each tank position. These are so I can wrap short bungees around the valves & hook them on the screws. This isn't really needed & I usually don't bother, but they can rattle a bit without it.
I did add 2 heavy screw eyes to the uprights. These are mounted just above the lower bar. I found that without a lower strap the bottoms of the tanks could slip around and on a hard turn they might slide out from under the top strap. Never had that quite happen, either, but I don't take the chance.