^^^ This. ^^^

Look at their stuff here: ORTLIEB - Produkte / Products /Produits

The bags are tough, durable, and completely waterproof. They're designed to mount on the bike quickly, easily, and securely. And you can take them off just as quickly. They're pricey, but you certainly do get what you pay for with them. I expect mine will make it through to my retirement in 12 years.

I prefer the older QL2 mounting system to the QL3. Near as I can tell, part of the QL3 system mounts to the bike. I need the flexibility of using other bags, and switching my bags from bike to bike., so it's QL2 for me.