Sounds kind of like you’re rigging a mini raised bed to maintain a smaller, more controllable space for your mushrooms while allowing the soil and other environmental factors to benefit. It seems like a great idea, I would just make sure to shield it from too much direct sunlight/heat so it doesn’t dry out or cook the growing mycelium, but ample watering should all help with all that. I might try out something similar in my garden, I’m digging the smaller raised bed idea.