Here is another idea,, for mounting a tool tube,,, and keeping it upright,,, and I don't have a tube to check the fit,,, but maybe it is worth checking. It would not take any fabrication,,, and could be removable. Mount the tubes on the rear slope of the inboard paniers. They would be about 15-20 degrees of vertical,, and largely in the lee of the panier. Of course you would have to be willing to drill your paniers,,, but then you could remover the tubes when desired. Kind of shades of Pinto, though,,,, Cat'