I used improvised grazing reins for a young student of mine I had a while back. My mare would put her head down to grab a bite every so often on the other side of the round pen by twisting her head and my student couldn't get her moving forward.
All I did was run baling twine from the bit, through the throat latch and tied it off tothe d rings of the saddle with quick release knots. I had it loose enough that it didn't effect her head carriage, she just couldn't get her nose to the ground. After a few lessons my student got more confident and as a result my mare quit trying. At that point I took them off.