My Viking's Magnetics tank bag "https://www.vikingbags.com/viking-survival-series-motorcycle-tank-bag" that sticks with magnets isn't holding onto the tank as securely as it used to.
I'm curious to understand how your magnetic tank bag worked
at all with the aluminum tank of a Brick because aluminum isn't magnetic. think Regardless, there are dozens of strap-secured tank bags in the marketplace although not all of them have a lithesome blond like the Vikings have to promote them.
Right now, I have a Coretech Super 18 liter expandable bag pictured on the Brick in my avatar here. It works acceptably and stays put although it doesn't have side straps for what could likely be total lateral movement control. I've never wanted one of those because I like the side profile of the tank without straps interfering with its looks. Mud and insects corpses don't bother me. The bag has two forward straps that couple to connectors on each end of a yoke attached to forward frame members, and one strap connector at the rear that coupled to one connector on a yoke looped around the left and right frame tubes forward of the tool box/Jetronic holder. It can be used as a backpack, too, to act as a survival means while you're scaling Mount Hunter, if you make it a quick roundtrip and can thrive on ramen noodles cooked with the flame of a votive candle and Cliff Bars sauteed with snow.
I had an excellent, expandable Eclipse bag for 7 years until its zippers wore out from lack of maintenance. Eclipse has discontinued most, if not all, manufacturing. RKA makes fancy, adaptable and expandable bags, and there are many other competent candidates out there.
Go with your gut, but avoid crashing upon the shoals where the marketing sirens sing.