I'd suggest putting down a rubber mat. That will at least keep "most" of the stuff in place. Phisics will always take over at some point if the load is not secured. You could also build some sort of box and strap it in. Then you put the small items in it. They also have these adjustable bars that will do the same thing as the 2x4, but you can put it anywhere in the bed and when not in use, just move it forward and clamp it down.
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1997 F150 4x4 Off Road, SuperCab, Short Box, 5.4, Royal Blue/Silver, K&N, Gibson SS, LT265/75/16 Wild Country APT's