UPDATE 15DEC2017: I have added a newer design that is slightly smaller to fit through a larger variety of spool holes. I have also made it better balanced (although it wasn't a problem before) and a little faster print since it's shorter (but can fit nearly the same amount of weight as before).

I made this to weigh down filament spools that were getting near empty. Due to the lighter weight and the greater instances of filament tangles as the filament runs out, my spools have been pulled off the filament rollers by the filament stepper motor. I found adding a weight on the inside of the spool held it down without adding much of any additional friction to the system.

The screw cap intentionally fits tightly to aid in watertightness. You can break it in by screwing and unscrewing it a few times.

To use, fill the container with heavy weight (I used 90 x 1.5", 5/16 hole washers on a 5" long, 1/4" bolt, which added 2lbs of weight). It does hold water without leaking but this depends on what settings you choose to print it with.

The filament spool weight goes well with my filament spool roller. http://www.thingiverse.com/thing:2250077