Aye Richard,
Used to use an Oxford tank bag (two tier type) on the Skorpion, no problems with it fitting and the waterproof cover that comes with the bag works. As the magnets don't 'work' make sure that the rear strap is set up firm and yet not trapping anything underneath the tank.
The bag in the ad. looks fine, but we have bought succesfully from Fleabay, both new and used Oxford bags. All the best.
My regards, Bill