I took one of these:
http://www.ebay.com/itm/Brand-New-NORTH-...=item51b35b101d
and painted it black. I put two holes in the back that match the fender mount bolts. The rest is just details. (weatherproofing the holes, a strip of foam between the box and the chrome, spacers to fill the holes in the sissy bar trim, a sticker, etc.)
It's water-tight, small, sturdy, and cheap. (I only spent a buck on the box.)