I haven't seen the movie and don't know what the shotgun is, but some company does, or did, make a flashlight that simply replaced the magazine end cap on the Remington 870.
You just unscrewed the magazine cap, and screwed on the flashlight that also acts as a magazine cap.
I would imagine that you could have most any machine shop make you a coupling that would serve the same purpose.
The Remington factory magazine extension uses a coupling that screws onto the magazine tube, then the extension tube screws into that.
It would be a fairly simple job to make a similar coupling with one end threaded to fit the magazine tube, and the other threaded to fit the threads on a good flash light.
It would be even simpler and easier to just buy a Remington factory magazine extension coupling from Gun Parts or direct from Remington, then have an adapter bushing made up.
One end would be threaded to fit the coupling, the other threaded to fit a flashlight.