go to your local pet supply house, to the kitty litter aisle, and look for Silica crystal cat litter, looks like translucent granules or spheres. Silica is HIGHLY absorbant and is used in gun safes and other moisture senstive storage areas to absorb moisture.
To use, fill a pair of your GF's or wife's stockings with the silica crystals. Use both stockings for strength. Tie it off and place in toolbox , it will absorb any moisture is a few days.
CHEAP, like 4 bucks. and youcan "recharge the silica when it gets wet, just bake in the oven at low heat.
Silica gel litter
Silica gel litter, a porous granular form of sodium silicate, has the highest absorbancy of any litter, and has excellent moisture and odor control.
Silica gel litter comes in two shapes: irregular lumps and small beads. Sizes range from 0.5 mm to 4 mm.
also consider relining the toolbox lid with waterproof foam strips, to seal out moisture