Kat's 3D device is what Coop calls the modified lenscap that Kat adds to a movie projector in Drive-In Me Crazy.
It is able to change things from the movie into real things.
Coop tells Dennis to destroy it while he takes out "the alien" (Creature from the cosmos) and (after Dennis reminds him) the "swamp monster" (Swamp Beast)
Kat first tests it on a train and it comes out small, things seem to retain the size they were on the screen, so using a big screen lets Kat make bigger things.