This is a simple collar I designed to protect drip irrigation hoses (and saplings) from the evil string trimmer. Since it was designed specifically for drip irrigation hose, there is a "tunnel" on one side to allow for underground hose installation. The slot on the back side easily stretches to allow for 1/2" diameter shepherd's hook posts.
Print in a color that won't detract from your landscape. I chose black to match my shepherd's hooks holding my hanging baskets.
It's a simple design, but I've included the OpenScad file if you need to alter it slightly.