To do this you will need a lot of gear including welding, cutting and grinding tools. Because I live in an apartment and don't own any of these tools I made it at Tech Shop. Check to see if you have a Tech Shop near where you live.
For my box I used approximately:
8 feet of 1/8 x 1 inch angle iron
8 feet of 1/8 x 2 inch mild steel bar
15 feet x 1 ft 16 gauge mild steel sheet metal
2 cans of Rust Oleum primer and paint.
Step 1: Locate the spot you want to place the box, measure and cut.
To cut the hole I identified natural lines that will not intersect with the frame or other important structures underneath the van and also worked with the ridges that are embossed into the sheetmetal floor of the van. I cut the floor with a portable Plasma cutter which goes through metal like warm butter but you could also use a high speed cutting wheel.
Measure out the hole you will cut and draw a line to follow with chalk or grease pencil. It is a good idea to drain and remove your gas tank before you do this step.
Step 2: Square up the frame
Step 3: Tack Weld the Frame
Once you have the frame tacked together you can begin to weld each joint where it makes contact with the other piece.
When you're done, grind down flat any rough spots or slag.
Step 4: Measure, Cut and Bend the Box Parts: Straps
for the straps that provide support I used 1/8 x 2 inch steel. The box is 10.5 in deep and 8 in wide so I cut each strap on the horizontal bandsaw to 29 in leaving about 1/4 inch extra for the bends. This box will be pretty rough so don't expect perfection and sometimes parts will not turn out. Buy a bit more steel than you think you will need.
After I cut the straps I measured where I wanted the bends and then used the steel press to bend 90 degree angles.
Step 5: Weld the Straps into Place.
Once the straps are welded on it might be good to double check that everything looks and fits right by dropping it back into the hole in the van floor.
Step 6: Cut the Side Panels
Step 7: Weld all the Side Panels
Step 8: Primer and Paint
Congrats you just made a Smugglers Box for your van. Next I will add a lid but that solution may vary depending on what type of flooring, if any , you will have on top of it.