Anything cage is bolted onto standard bottle cage bosses on your bike, but allows you to strap a myriad of things onto it. Nalgene, oversize bottles, tents, dry bags, sleeping backs, well... anything! Great for bikepacking!
I looked at the Salsa Cycles Anything Cage, as well as the Blackburn Outpost cage as my inspiration. This one is a 3d-printer-friendly design based loosely on the dimensions of the Blackburn.
Update: Printed on a Prusa Mk3, in standard PLA. No issues during print--I will note that this object feels quite sturdy and rigid, with the exception of the bottom shelf. I may remix with a more rigid design in the future, but for light loads I don't expect a break.
Keep in mind that the STEEL anything cage from Blackburn recommends no more than 6lbs. While that one could probably hold a LOT more, I wouldn't recommend more than 6lbs for this design.