Ahaha, poop on the sofa! xD That cracked me up!

I recycled my old carboard boxes at the bottom, and then added the old autumn leaves from the trees on top of that, and some smaller tree branches before the adding the soil on top. It worked alright, but if I would do it again or make more of the boxes I would put at least a double layer of carboard, because I feel that some grass from the bottom broke through the only one layer. Carboard deteriorated over time with watering and rain. Straw bales might work just fine too! Curious to see what you will end up making there! :)