Funny, I was actually talking about this today. I 2nd paulages, ammo boxes make excellent panniers. I have seen chopper guys make them out of 20mm boxes, but 50 cal might work too. My local surplus less them for $25 and $15 respectively. The 20mm should be big enough for a helmet, clothes, laptop, etc. I think it would be fairly easy to fashion brackets from stiff 1/8" by 1" steel strap using the passenger peg bolts and rear fender bolts as anchor points. From there simply bolt the boxes to the steel and you should be in business. If looks are important, paint the whole thing black using automotive laquer.