This is an original design based mostly on philwaud's concept. I wanted more secure threads without the slit extending through the top, so this is what I came up with.
The hole is intended so you can "clip" the bag into the top or bottom recess. Also, it's easier to feed the bags through the hole before you put the spool in rather than trying to fish them out through the hole.
Update: Added the original .ipt files as well as .stp if you'd like to modify the design to better fit your leash.