Get a USAF Flyer's Kit Bag for less than $20. There are US Air Force bases in Germany so these will be available near them in used excellent condition. I think the Leftwaffe uses them too. They will hold my full 3 piece Bushwacker pannier set and the handlebar bag. You just stuff it all in there. They have huge zippers and weight about .5 Kg. I bought 12 used ones 20 years ago for $10. I'm still using the third one (the rest are in the shed).
For examples, see: http://www.vtarmynavy.com/air-force-crew-bag.htm