A helpful lid that you can screw onto a standard mason jar to create the perfect fruit fly trap.  Put a bit of liquid bait inside (just about 5 mm deep), and place where it's needed.  

There are many commercial fruit fly liquid baits available, or you can just use a bit of apple cider vinegar or wine.  The flies are attracted to the smell of the bait, and then after entering the jar are unable to find a way to fly back out.  You can also add a few drops of dishwasher detergent to the bait if you'd like, which will cause the flies that land on it to drown (as they get caught in the surface tension of the liquid.)

This is an easy print that requires no supports (it's designed to be printed upside down, as the STL is oriented.)  Would recommend slicing on your own, but I'm also attaching the ‘gcode’ I used to print the one shown in the pictures on my Prusa Mini.