Comment by Matticus_Rex

Here's a thought -- there are instructions in various places for mosquito fan traps using a box or other sturdy fan plus mesh on one side.

Do that, but paint the fan (with blades) blue. If more movement is needed, you could probably do something pretty simple with a plastic plate painted blue and hung about a foot from the fan intake.

Cleanup isn't great, but it's better than sticky traps. Spray the mesh down with a 50:50 water:alcohol solution as needed to kill live flies in the trap before dumping.