Necessity is the mother of invention. Back in the 1970's I wing nutted two small cheap suitcases to the back of my BSA one red one blue painted them black. They weren't waterproof but they did the job for a while until they started to disintegrate. :hehehe I used to have a make up case as a top box on the Brick when I first got it, I bought four cheap from Zelows they didn't have the combinations, sold two kept two and I still have one. A lot of mates wanted to know where I got them from I paid $20.00 Au each, a bargain.
Regards Martin.