Countersink and screw it in. I use flat bar for the sides of my box. Angle irons aren't square through the bend so when you lay them on you have to screw them in from the top anyways. If you screw them in from the sides it'll push the tops up as it tightens.
So I can counter sink a flat bar? I didn't know how easy it would be to screw through metal like that. I see what you mean, I'll definitely try that.