This is a lasercut pellet reloader for Walther .177 8-shot magazines, though you could just extrude the two shapes and stack them for 3D printing. I think they're made by Umarex, so probably works with that too. I cut them from 3mm MDF and glue the two pieces together. The central hole is for a woodscrew to act as a lifting handle.
Put the assembled stack in an empty pellet tin, and push the magazines into place. Add enough pellets to form a single layer, and shake the tin back and forth. The pellets should automatically seat in the magazine.
It works fine if you only put one or two magazines in.
Lift the device out by the woodscrew, and put it on a flat surface. Push the pellets in with whatever seating tool you use (I have a biro which fits just nicely). Pop out the loaded magazines and have some safe fun.