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Sorry, I should have been more explicit. The trick for this trap, and for a Malaise trap, is to place it across a 'fly way'. This usually means across a woods road or path or alongside a hedge. In my photo there is a row of trees and bushes on each side of the path and a wood behind the trap. The path comes comes in behind and to the right of the trap. The open hole is a doorway that can be closed with a zipper. Also note the orientation. The trap is placed such that the doorway opens away from the sun (usually North, Northeast or East), flies entering this doorway will then accumulate on the opposite screen on the South, South West, or West. Standard Malaise traps should also have the collecting head orientated towards the sun. When one sees a choice fly in the trap, one steps inside, and zippers-up the door. I'm certain many flies escape the trap, but for the effort and cost it still seems a reasonable system. When using the trap yesterday I also collected flies with a hand net but saw less than half-a dozen flies, compare this to the 50 collected by the tent.
I'm planning to place a big black shiny ball inside to attract tabanids.