Please critique my fall clean up box for pick up

[recycledsole]

hey guys, i would appreciate anyones help on this.
i want to build walls on my pick up truck for leaf and debris collection. so i can haul more things more securely.
i have a toyota tundra with 6.5' bed.

i will use 4 2X4s in the stake holes @ 4' each.
i will use 1/2" plywood walls @ 4' high. (is this too high)?

the things im concerned about are
1) the box is attached to the truck only by 1 lag bolt in each 2X4. doesnt seem secure enough. is it?
2) the 2X4s going in the stake holes being modified to a smaller size to fit the opening. will they break? is there metal - square steel tubing?

do i put the plywood on the inside or outside of the 2X4? i think outside would be better

thank you!