This is a legit method for building small things like that where you can use the brick-like shape of the bag when it’s hardened. You just need to make sure the concrete gets soaked all the way through with a hose or rain.

https://www.core77.com/posts/80454/An-Easy-Way-to-Build-Retaining-Walls-Leave-the-Concrete-in-the-Bag-Stack-Like-Legos-Wet-With-a-Hose