First, make sure you have the sliders at the rear of the screen in the closed position blocking the power cord holes.
Even when properly closed they still seem to find a way out by sneaking under the frame of the screen if it's not seated properly. My twig roof prevents a proper seal so I'm either going to stuff some foam weather stripping around the inside crack or trim the crossbeams so they don't interfere with the seal.
So far it really hasn't been a problem but it is annoying to find a few loose flies now and then.
I let a horde of spider loose to take care of the flies but they kept biting me so I lured crows and starlings in by spreading bread over the floor and leaving the windows open. Unfortunately all the bird poop is a real chore to clean up so I brought in a bunch of stray cats which drove my allergies crazy. I bought some rabid dogs online cheap to drive out the cats but now I'm stuck up here on top of the refrigerator with my laptop, sneezing, covered in bird poop and spider bites. I guess the flies weren't so bad.