Dog poo bag holder. Holds standard roll of dog bags for those long walks past that one neighbor's house who turns the sprinklers on when he sees you coming. Alternatively; use as a tiny human being poo bag holder for when you're backpacking through the Serengeti, and your infant child drops a deuce. Happens to me all the time, can't take them anywhere.

The lid has a feature that will "click" into place when its screwed in, this holds it in place and the handle is always on the opposite side of the hole.
Included is also a version without the bag hole on the side, in case someone wants to use it as a regular container or box, hopefully completely unrelated to poo.

I used a cotter pin and a large standard key ring. A 3/32-1/8" or 2-3mm (diameter) x 1" or 25mm (length) should be fine. They sell 3packs for around $1 at Lowes, Home Depot or equivalent hardware store. Also a thick paper clip should work too. I bent the ends back to hold it in place, then cut them down to size (see additional pictures).
Key rings are super common. you can find them in the same place you find the cotter pins, or go to any home and garden trade show or job fair where they hand out the free ones for marketing. That's what I did.