Don't know whether they're available in Europe, but I use Wolfman roll-top panniers slung across my tank and bungied to the nerf bars on my KLR. These are not little nylon tank panniers, but rather nominally full-sized bags designed for rear racks. I use a single black rubber bungie on each, having tried and discarded various other attachment methods. I've put about 40,000 miles on the bike with this configuration, in all sorts of conditions and weather.

I'm sure you could do the same with a pair of H.D. roll-top dry bags: sew some 2" straps in appropriate lengths to cross over your particular tank and crash bars, maybe including sliders for adjusting length. You might find a better solution than my bungie straps, but if you do as I did make sure to get high quality bungies; the cheap ones disintegrate in UV. And don't forget your mother's admonition, "You could put an eye out with that thing!" when stretching them tight. The metal hooks can definitely do some damage if embedded at high speed into human flesh.

FWIW, my Wolfman panniers have never been waterproof despite costing well over a hundred bucks; they collect water inside but don't let it drain out, which makes for some absurd situations when riding through rains all day. Next time I'd buy some materials and have a sailmaker sew something more suitable for me, including grommeted holes for drainage.

One more caution: big front panniers like these can hold a lot of stuff. For handling reasons, they need to be kept fairly light, so resist the temptation to load them with tools, parts, oil bottles, food, etc.

Hope that helps.

Mark