Magnet Receptive Sheeting
I just got some of the plain (very dark brown) magnet receptive sheeting and very pleased.
It is very easy to work with and receives a roll-on adhesive well.
I use it, glued to MDF, for movement sabots in wargamming where the bases have magnet vinyl sheeting on their bottoms. The magnetic pieces stick to the receptive sheeting sufficiently to be completely useful in my application (you can hold them upside down without the pieces - 10mm figures on wood bases - falling of).