An aiming tool for picturing the “ghost ball” in pool.  Line up the long red line with the pocket you want the object ball to go in, go back to the cue ball and line up your shot with big red ball in the center.  Remove the tool and shoot.

Designed for a standard US ball (2.25"), but should be scalable to any ball.

I based this off a gadget I saw online, and thought I'd print one.  The fins on the model don't really do much, for me, so I made a reduced version that minimizes waste with filament changes (actual waste isn't that much but it took 8 hours to print).  But I had a 0.2 nozzle on at the time, so I re-sliced the model with fins and a 0.4 nozzle and it can print in 2 hours or so.  That said, the 0.2 model came out very clean.

Ignore the differences in the curved section on the comparative photo, they were early prototypes.  The final model minimizes contact with the object ball so it can sit snug.