Here's the deer fly trap I made for about $5. I found a large blue ball at Walmart for $2.99 (the ball is maybe 2 foot diameter), constructed a twine harness, which I painted blue to retain the effect of the solid color that deer flies are attracted to, and suspended it between two small trees so that when the wind blows, the trees sway and the ball with them (that supplies movement, which attracts flies). Finally, I coated the top with sticky Tanglefoot. I already had supplies of the blue paint, twine, and Tanglefoot, so the trap didn't cost much.
It works! After only three days, there are 30 deer flies trapped by the ball, and not one deer fly swarmed me while at the garden for the first time in weeks. Success!
The trap was also catching black flies. Yes!!! As far as I could tell, no beneficial insects were getting trapped (like butterflies). So this trap seems to be effective. I also checked a couple of small mosquito traps I made with plastic soda bottles, brown sugar, and yeast, but these do not seem very effective so far, possibly because too much liquid was in the bottom. When the yeast started creating bubbles, it just foamed up and blocked the opening.