This Instructables shows you how to reinforce a standard cardboard box using wood laths fixed in place with staples.
Step 1: Materials
The first thing you need is obviously a cardboard box, I recommend a reinforced one (two cardboard layers), since you are obviously intending it to be able to carry a lot of weight.
The second one is to get the wood laths. All of them should be wider than the staple you will apply. To build the cross in the bottom, the lath under the tabs should be slightly shorter than the one over them. It is possible, of course, to order them all the same length and remove 1 - 2 cm with a hand saw.
The laths on the corners should not be longer than the height of the box (same length recommended). Their thickness will depend on the staple applied. I have used 8 mm staples, so my laths are thin.
As already mentioned, you are going to need a staple gun and compatible staples.
Step 2: Bottom Reinforcement
To reinforce the bottom, I have placed two laths forming a cross. They are fixed in place with staples. I must stress that stapling it should not be critical since a lot of weight will hold them in place. If you want to prevent them from moving in case the box is handled upside down, than staple it.
The lath over the tabs should be stapled on the bottom lath. You will need long staples to accomplish that. In general, staples do not adhere well to the cardboard.
Step 3: Corner Reinforcements
The corner laths should be stapled from the outside of the box. It is generally a two-man job, because someone will push the lath (preferably using a plank) against the cardboard while another person will staple it from the outside. You should place it close to the corner, since it is, from my experience, the weakest link of a cardboard box.
Step 4: Test It
As a final step, try to push the laths free with your hand, they should not move. If they detach, repeat the stapling job, you have probably misaligned the staples.