https://www.horse.com/item/fly-rid-plus-spray/E006407/
I bought this stuff to keep mosquitoes off my goats, after I saw them using it to keep flies off a cow. It can be sprayed on the walls. It also works on lice and mites and labeled for use in chickens with no withdrawal time for eggs... Under $8 lasted me 2 + years with 82+ birds, 2 large goats, and 3 medium dogs. In the equine section at the feed store... active ingredient is permthrin.
Also, you can make homemade traps using a water or soda bottle with it's top inverted like a funnel cone... a little raw meat or oddly enough an apple slice to attract them. Look for some other versions online.
I won't personally use DE... but you might be able to use some PDZ to keep things dried out a little... it might help.