[Boltxxx]

Most POS skimmers that are not in the form of POS malware the physically ones tend to be complete overlays.
Theoretically what you need is something that records the pinpad inputs which is done with an overlay....and a form of skimmer that collects details on inserted cards.... im not exactly sure how you would do it but it cant be easy anyway because its like infront of a person and on CCTV.... which is why most people who target POS devices tend to use malware.