[UPDATED July 18 2018: I added an updated version that can be printed in two halves, no supports needed. They must be glued together, but to help with this the four holes are the right size to insert four 177 pellets to align the pieces. The "Pellet Belt Stacking Storage - 2 piece.stl" file is what this is.]
This is a stackable (they clip together) storage tray to hold the SIG ASP/RPM 177 cal pellet belts.
The belts can be pre-loaded but they need to be kept clean and are kind of floppy and awkward, so these trays (which are stackable) are very handy for transporting loaded belts.
There are two versions. One has ridges to hold the belt better, and the other just has a flat bottom. I prefer the ridged one, but the flat bottom will be easier for so-so printers to print because the layers are smoother.
The companion "thing" for this is my ASP/RPM Belt Loader Tool: https://www.thingiverse.com/thing:3001285
If you want to model your own ASP/RPM belt things, you might find my CAD model of a belt cutout handy: https://www.thingiverse.com/thing:3001292