Best tank bag I've ever had has been Enduristan.
enduristan.co.uk
They're strap fastened, not magnetic. Not sure if they'd fit the Rocket but they're semi-soft flexible bags so I'm sure it would be doable with Rock straps.
Quality bags aren't cheap I'm afraid. Best thing about Enduristan is they're 100% externally waterproof. No rain cover or waterproof liner required. I think they're good looking and they come in a range of sizes.
And it's a very small detail, but I like the fact they have a bright red lining so when you open them you can see to find things.
I've had one for years on my KTM and it's been exceptional.