The same sort of stuff used to make fridge magnets, in a convenient pack of fifty pre-cut pieces, each 40mm x 20mm. These sheets are self-adhesive and no glue is required (just peel off the backing sheet to expose the sticky side) and it can be trimmed if necessary with scissors or craft knife. Very useful for lining sticking on existing bases for storage in steel toolboxes, or to allow steel-lined movement trays.