I go with box files, lined with steel paper. Then I put magnetic sheet on the bottom of the stands.
It's a bit tedious to cut all the magnetic sheet up, and stick to the bases, buts it's not hard. It's important to clean off any bits of sand or other stuff from the bases.
It's amazing how the number of box files needed keep growing. Once upon a time I had 1 box file and labelled it Elves and Orcs. How naive this was! The Elves alone now fill at least 3 boxes, Empire much same, Araby just over a box, the poor Orcs still only fill one box, then there is nature which now spills over into its second box. And that's just the painted fantasy stuff...
I also have several metal drawer units, the most compact ones are bisley ones which the drawers are about 30mm tall, but these are expensive. There are also some ikea ones that are much cheaper, but the drawers are very deep for 10mm figures.