I use magnets on the base of the figures and have cut sheet metal to size (though I use it as a carry board to move my army around rather than as movement trays).
You might find that is cheaper than magnetic rubber sheeting and also adding magnets to the bases/using magnetic bases.
I don't know about the hardware stores around you but my local one will cut things up for you if you ask, so they should be able to cut the metal plates to size.
Then add a layer of cardbaord to the base to make the magnet closer to the metal sheet when you glue it to the cardboard plinth within the base.
I got a metal sheet of about 15" by 8" for about £5 (I went and tested that it was magnetic in the shop as well

). I got a bag of magnets from maelstrom games for about £4 for 40 magnets (I imagine you can almost certainly get rare earth magnets cheaper elsewhere however).