We use rubber stoppers.
https://www.amazon.com/200Pcs-Pourer-Spouts-Covers-Bottles/dp/B07KP2QPK1/ref=sr_1_9?dchild=1&keywords=liquor+bottle+cover&qid=1634167289&sr=8-9
This, plastic wrap over drains, and a floor fan knocked out the flies in just a few days.