I built a box once to use on a friend's boat trailer which was modified to add canoe racks.

I attached to box with bolts to the wooden boat supports/rails/whatever-you-call-'ems on the trailer.  3 bolts on each rail.

The box was 1/2" plywood, top-opening on each side.  I just measured the space it needed to fit in, figured out the proper height to contain the packs in the box, got some steel 90 degree braces, hinges, and hasps, plus a can of the cheapest outdoor latex paint I could find.  It was cheap and easy.  The only thing I wish I'd done differently was used 3/4" plywood.  

The box lasted for several trips before I gave it away to a friend.  Not sure if it's still in use or not, but I figured after 2 trips I'd gotten my money and effort's worth out of it and anything past that was bonus.