Not sure card stock would work that well unless it was really sturdy, since I think it might tend to buckle with repeated pushing it up and down into the card detector, and might slip back down into the card detector when you move it to the "up" position when leaving the room. If you want something disney themed, I would suggest finding an old plastic card you don't need and laminating a disney image on top of it.
Another option is there are lots of cute disney-cruise-themed light cards on Etsy available for a dollar or so. We bought these for our FE group last cruise and got an extra one for ourselves because they were so cute.