Right after watching the trailer for “Star Wars: The Force Awakens” last April, Ken Landrum began building his own Stormtrooper gun. From his home in St. Louis, he cobbled together images of the “blaster” featured in the ad, and then used software on his personal computer to design nearly 40 separate pieces to be 3-D printed and assembled into a near-exact replica of the Walt Disney Co. prop.

“My goal is to make it better than the studio did,” he said. At a minimum, he has done it faster: Mr. Landrum posted photos of his design on a message board for 3-D printing enthusiasts—some eight months before the movie premieres and five months before most official Disney toys hit shelves. As the movie’s opening gets closer, fans have filled his inbox asking for the files needed to print their own. Mr. Landrum said he handed out more than 100 in one week in mid-July, recently deciding to start charging $55 a file. “It’s gone haywire,” he said.

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