Bottle Fly Trap for Mason Jars

Fill 1/3 of the jar with water and a add a scrap of raw fish or raw meat. Screw the cone or tubes part in with the lid ring, them pop the top cover into the four small holes.
Within a few days, as the meat decomposes, the flies will start filling your trap! White plastic seems to work best since the flies are attracted to light and will head up to the underside of the cover and not the light shining through the holes. The top cover stops rain from filling the jar.

Pictured is my cone trap after only a week or two. And, yes, that is nearly a full quart of green bottle flies! Gross!

This is for wide-mouth Mason jars. I have included the JSCAD file in case you want to adapt it for small-mouth jars.

This is best printed in ABS or other heat tolerant plastic if you live in a hot area or put it in direct sunlight.

Update: Version 2 (V2) files have a support and hole to allow you to string a cord through them and and hang the jar from overhead.
String the cord through the cover, fit the mason jar lid ring on the cone portion, then string the cord through the hole in the "X", tying a large enough knot in the end to prevent it from slipping back through the hole. Screw the lid on the jar with your bait and hang it from a convenient branch.