I have found a fairly effective way to mount a truck box in the bed. The following information should be verified on your truck and box before using. Measurements and comments apply to the 2004 Double Cab only, but the concept may work for your truck as well.
First, I found that the deep and full size truck box I owned (AD101 by Adrian Steel) was so wide that it barely fit into the Tundra bed. It would not fit at all with the official Toyota plastic bedliner in place. So, the overhanging section of the box that rests on the bed rail cap has no room for the typical J-bolt mount. Your box may be narrower, and your J-bolt may fit, I realize. But, the Adrian Steel box I own is so large that the J-bolt method would not work.
In addition, the Tundra forward bed tie-down loops are mounted in such a way that they interfere with the standard mounting location for the box, pushed all the way forward in the bed. Since this looks odd to me, I decided to remove them from the bed. The loops are affixed with M8 x 1.25 bolts. A T40 is used to remove them.
I sprayed the bed with a Line-X
liner, and had the shop plug all tie-down loop holes prior to spraying. The
liner ensured that the bed paint under the forward loops was protected from corrosion, as after taking the loops off I saw only primer there. I had to go with Line-X as the box would not fit with the standard plastic
liner in place.
So, with the forward tie down loops gone I saw that I could bolt the box to the bolt holes in the bed, and properly secure the box in an unobtrusive way. I measured the double cab bolt hole locations carefully and transferred these to the truck box. I allowed for a thickness of closed cell neoprene foam tape under the box overhangs--you get this at Lowe's. The key dimensions, again, for a double cab, are as follows: the upper bolt hole is 10 3/4 inches down from the box overhang underside surface, and 1 7/16 inch rearward from the box wall that goes against the bed forward section rail. I had to measure from the very upper edge of the forward rail, not the bed wall itself since there is a bit of an overhang in the bed forward wall. I centerpunched the hole, drilled a pilot hole very small, and checked with a stiff wire to see if the hole was aligned with the bolt hole. After verifying, I drilled the box with a 3/8 drill to give clearance for the M8 bolt.
I went to the hardware store and purchased two M8 x 1.25 x 50 mm bolts to mount the box. The 50 mm length is needed for this particular truck box. If your box is narrower, a longer bolt would be needed. I also found a couple of fender washers and a rubber fender washer to seal it all up. The bolts went in fine, and the box is securely mounted.
So, a mod that feels good. The box isn't going anywhere, and yet can be easily removed for loads of mulch. Hope my write-up is of some help. Let me know if you have questions.