[attachment=0:2ccervp9]img069.jpg[/attachment:2ccervp9]
Something like this, made out of 1" flat bar, may be a start. Measurements so that it fits across the bike on each of the side mount bolts. Then, maybe, the Box, or the mounting plate can be fixed to the two pieces. The height of the side pieces of the brackets is to suit the height you want the box to sit at.
The only tools required.....tape measure, pencil, bench vice, hacksaw, file, hammer, drill.