Dunno if Rhinowalk do spares, but Kriega do. I had a US30 in my hotel room when my bike with the frame straps for it got nicked. Kriega sell a frame strap set. Shame Triumph don't do the same, as the frame straps for the tank bag were only available if you buy a new tank bag. Also, as pointed out by a mate, if you have multiple bikes, you can fit frame straps to them all and move the Kriega pack from one to another.
[Edit] Rhinowalk do separate strap kits.