The legs on those look a bit weak to me.
The wind effect on the box sides can be enormous during storms, use 2 foot rebars banged into the ground 2x min with brackets into the wood to get fixations down into the ground. We build our own here in Germany by cutting weak trees out of the woods, debarking them then build them on the barn or garage floor as 4x sides then assemble them at site, friends are needed to avoid accidents though.