Used a jigsaw to cut the 5" notches in my 2 foot 1 x 6 inch.
Then glued and screwed the notched boards into the backing boards.
Then screwed the backing boards into the arena wall
Two-inch lumber would be sturdier for sure. I just had 1-inch lumber lying around and it's easy to cut with a jigsaw. I literally made these in about half an hour.
My utility hooks (from a recycle place) were more C shaped but these would be great.
I bolted the hooks to 2x4s, which I then tied to posts (another use for baling twine). (I didn't bolt them on because one post was a T-post and because I didn't want to presume to attach things too permanently.) The 2x4s have a "foot" you can't see being weighted down by heavy wood poles.