I built a box to fit in mine. The front row (next to the cab) will hold four 5-gallon buckets - usually peat moss, drags, etc. It has an elevated shelf and has storage under it for extra tools, winch, etc.
The middle is narrow and usually has stakes, drags, chains, dirt hole drill, big hammer, etc.
The back (next to the tailgate) will hold four 5-gallon buckets (set bucket, lure/bait bucket, peat, etc.)
Once traps are set I'm usually down to just a few buckets with some straw in the other holes for non bloody critters like cats and fox.
Mine is a crew cab so on set day I have two kitchen trash cans in the back floor board with traps. A tub goes in the back seat for extra items on set day but has straw in it on check days for the bloody critters.
The picture above was on trap pulling day so there were no buckets.