I have one of the HD style boxes where the rear peg hanger was. It has 2 mounting holes that line up well with the peg hanger bolt holes. I made a brace to fit the rearmost hole in the box from a bit of 3/16 thick strap iron. The forward end of this brace bolts to the upper peg hanger bolt hole.

Let's hope there's intelligent life somewhere in space 'cause it's buggar all down here. -- Monte Python