I feel the same as you. Not crazy about attaching them to my knees, but after going for a ride with them once I now won't go without them.
Magnets wouldn't hold. Too many variations to consider to make it so people could attach it to their motorcycle. Not sure how effective they would be mounted to the tank, if they were positioned for your feet to be on the standard pegs and you moved them to the highway pegs.
Iv'e been on the interstate doing 70 - 75 and they hold up okay.