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I have not had experience, but I would suggest fixing fine netting around the rotholes, to this fix an upturned plastic funnel, and to the uppermost (thin tubular) end of the funnel fix a collecting container - something like a small plastic bottle shoud do. The flies should crawl up towards the light (as in the malaise trap principle). You could use ducting tape to join these things together, but be careful not to leave any sticky areas exposed on the inside of the apparatus, that flies could get stuck on.

I hope to try this method myself sometime.