Wobbly side box

I bought a 24x24x60 toolbox for between my spread axles. I fabbed brackets out of angle iron, and used a piece of flat strap to make them a "triangle". Just regular brackets. Anyways, now I've got everything loaded in there (probably 200 lbs), I can shake the box up and down. If I keep at it, I can really get it moving. Maybe 3/4" of movement at the farthest edge. I'm worried about metal fatigue as I'm driving down the road.

Are your boxes solid, or do they wobble too?

 

No, they're dead level, square, and secured. I've got the bottom of the box bolted down with four 3/8 bolts. The back of the box bolted with four more 3/8 bolts. The brackets are bolted to the trailer frame with four 1/2 bolts.

The toolbox and brackets aren't moving independent of each other. They're all wobbling together. Now they don't just wobble "easy". I have to push them up and down, and keep at it for a few seconds before I really get it moving good.

 

I guess a better way to explain it is all the mounting points are solid, but the whole apparatus is kind of flexible. Springy.

 

LOL, I'm just worried what's going to happen when I start going down the road.

 

No, two inch. 1/4" thick. And the 1/2" bolts holding it to the frame are 6" apart. (that's about the best I could do)