I chopped two 2 1/4" pieces of angle iron and smoothed out the edges. Then I cut five pieces of 2" flat bar; two for inside the box, two for the sides and a longer piece to go across the back.
I hammered the two side pieces in the vice so that they had a slight curve, then welded them to the longer piece. I ground off my ugly edge welds and then feathered it back to a nice shine. Then the side pieces were welded to the angle iron (don't weld on top of plywood; I did that because I got impatient and didn't want to clean off the welding table).
Next holes were drilled for the bolts. The holes were marked out onto the two inside pieces and they were drilled out too. I didn't go from smaller bits to larger (recommended) but I did use oil when drilling.
The bracket got hammered around a bit until it fit again, then I drilled out holes in the tool cart. I sandwiched the plates and put the bolts through, tightening them with an impact driver.
The final stage is to hit it with a shot of black spray paint - makes it look lovely! And there you are! If you also own one of these carts it may be worth making one of these brackets before it cracks on you on a job - a stitch in time saves nine!