Despite Fly Strike being, on the rise, in the two years, of keeping sheep, touch wood I have never had a case of it.
What I use is a Garlic lick bucket, and once a week, I put a garlic powder, over sheeps feed.
The neighbours who rear sheep, have been hit hard last 2yrs.
My pigs, goats etc all get it.
And despite getting a bit of dirty bottoms, when put on fresh grass. No maggots