The only thing that springs to mind is to drill a cavity in the bottom of each base and insert some metal and place a magnetic sheet in a box. Seems like a lot of faff though.
A lot of faff may be a long, tedious road…but does have an advantage. If you put a magnet (.62″ dia) into the base instead of metal (as Mr.A stated) you can use any old metal for storage,
terrain/bldgs can accomodate figs by simply having metal sheet incorporated into their construction,
and tokens/markers can be attached in the same way.