Thyme oil spray
You can buy thyme essential oil online and prices vary depending on the purity of the oil. The scientists in this research said they believed thymol was the active ingredient that was toxic to red mites and that it was likely the higher the thymol level, the better the kill rate, but more work would be needed to assess this.
1 litre water 25ml thyme essential oil
Mix and spray onto coop surfaces, making sure to get it into wood joins. If possible, remove perches so you can get spray into gaps where they butt up against or into walls.