I came across this on facebook and thought you might be able to use it or at least get some ideas:
Built a horsefly trap 3 days ago. Day 1 killed 146 horseflies. Day 2 killed 375! We either have a terrible horsefly problem or it's a great trap.
UPDATE: The 7 day total was 1,697 Horseflies killed! They are big (1" long) and easy to count. They need to be cleaned out every day anyway so they won't rot. How it works is simple. Horseflies are visual hunters. From a distance the black bag looks like something to bite, they do a fly over and are deflected into the soapy water and drown within a few seconds.
UPDATE: I've added a couple of pics since many people have been sharing this and asking for plans. The leg configuration can be whatever you like. I just made mine so it would be easy to move and change the bag. One last tip: The plexiglass needs to be keep clean and free of scratches. We have high humidity here so I squeegy the dew off the plexiglass every morning. The 3rd pic is one day's catch! I've quit counting. ;-)
UPDATE: I just realized the pic of the HORSEFLIES was blurry so I replaced it with a clearer pic. Also added a 4th pic of the saw cuts in the 2x4 that holds the plexiglass. They are at 45 degree angles about an inch deep. The plexiglass fit tight enough that I didn't need to fasten it in any way. The bag I used was a 55 gallon trash bag from Sam's Club. It holds about a 5 gallon bucket of water with a big squirt of liquid dish soap.
UPDATE: Make sure and set this in an open area where the horse flies can see it from a distance. You may need to move it around until you find the "sweet spot". Clean it out with a cat litter scoop or large slotted spoon. Also I would love it if you would pm me pics if you do built it.
NOTE: Be sure to paint the top board a dark color so the light color of the wood won't reflect in the plexiglass and make it visible to the flies.
UPDATE: Dan Fowler from Tennessee sent me a video of his set-up catching a Horsefly. He caught over 6,300 in one week!