I just remembered a fly-catching method my dad had when I was a kid and had frogs to feed.

First get a large glass container, wide-mouthed. I can't find a picture of it, but it's kind of like a goldfish bowl with flat sides so you can lie it on its side.

When it's on its side, put a small piece of meat - like a piece of stewing beef - at the back of the jar.

Put two strips of tape in a cross shape across the mouth of the jar.

The flies can get in, but they can't get out.

Hope you have success with those pesky things soon!