Arcade power supplies are designed to be installed inside of arcade cabinets, where they are safe from hands, kids, and pets. Supergun users however often use arcade PSUs out in the open, where the exposed power terminals pose a serious safety hazard. 3D printed terminal covers solve this problem by encasing the terminals in plastic, protecting from exposure to dangerous live line voltage.
QCRetro designed a Happ 15 Amp terminal cover: https://www.thingiverse.com/thing:3791090
That design is a remix of one for the Min Hong MH-16A: https://www.thingiverse.com/thing:3464794
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