I agree that it's probably the exposed liquid that's attracting the house flies. I think the repellent properties of the BSF larvae work best if concentrated in a more closed container. One downside to exposing the tea is that it will attract females to lay outside of the unit, but it looks like you're getting plenty of eggs laid so it shouldn't be a problem. I have BSF laying in several areas because of my different tests and they still are barely noticeable. They are the ultimate respectful visitors.
I think I've mentioned before that I want to test some methods of running a BSF unit without a drain. I may get a second BioPod and try sealing off the drain hole to test the different approaches.
I also had a thought about a way to regulate the heat during these hot summer days. I'm thinking it might be advantageous to withhold feeding the larvae until sundown so they can do the majority of their work in the cooler evening hours and then rest more in the daytime.
