Stick it to the door frame, high up and you should be fine. There aren't a lot of things that are damaged by magmets... Mainly magnetic storage devices I would say, credit cards are the most sensitive ones hence high up in the door frame, away from your pokets or bags when going in and out. In any case, in years working with audio, moving strong speakers very close to my body I've seen only a few damaged cards in my pocket, two or three at most.
JS