I’d absolutely put something up along anything more than 8’. One in the middle would be fine, I have a metal brace in the middle of my 12’ spans. But you could do 2 of them at the 1/3 and 2/3 distances.
Do you have a table saw? If so, or could borrow one, just lop off the stall-facing corners. Of if you’re hand enough with a skillsaw, you can do it that way too. Many of them allow you to angle the blade a bit.