My wife has one he used to place on her VLX600 tank. Not real big, but after a year of using it, taking it on and off . . . the tank had surface scratches on the clear coat.
I have the Bagster tank bra (not the tank cover) & a bagster "Design Tank Bag" which convrts into a rucksack as well.. Bike Johnny, our sponser does them.
Before that I had an Oxford magnetic tank bag which I used with a non slip blanket material & I never got any marks on the tank. Used it for 3 years on the Speedmaster & then the Rocket
It's superb. Strong magnets but with a headstock loop as well. I've used it for 2 years now with an Oxford non slip weave between it and the tank, with no marking.
Don't know if it's available in the UK - bought mine off e-bay.
I use a magnetic Oxford one with a mat underneath after ruining the tank on a previous bike (ZRX1100) when one of the magnets picked up a coupla small screws in me garage and I didn't notice. Doh!
It converts to a rucksack and the base can be used as a map holder.