Fishing can be frustrating if you have trouble casting. And it is not a great idea to throw hooks around your yard or the local park while practicing your cast. Thus the reason for a practice plug. It enables you to practice casting without implanting a hook in your neighbor's eye.
This practice plug has a different design than most; it is threaded to take a Coke bottle cap from a standard 20 oz bottle. This gives you the advantage of being able to add weight to suit your needs instead of being stuck with the weight determined by the manufacturer. Use whatever you have lying around: fishing weights, lead shot, BBs, very small rocks, cider, gravy, cherries, mud, churches.
It weighs 1/4 oz (~7g) empty with the cap. It weighs just shy of 2 oz (54g) filled with no. 8 lead bird shot.
Th thread design was taken from nearadyn's "Match case with pop bottle threads" (https://www.thingiverse.com/thing:690488). If you haven't seen it yet, it is a nice match safe (For you city folk, that's a container used to keep your matches dry).
For fun, try saying "Threaded Casting Practice Plug" three times fast!
REVISION 21 July 2019: Added a simplified file that reduced the amount of faces of the file by 75% (and reduced the file size by about that much, too). I got a report of trouble with the file; hopefully this helps.
I used PETG. PLA may be too brittle for repeated use. But, hey, print the way you want.