To Prepare Your Own Site:

The best base to use for a utility shed or gazebo is a flat and level 3/4" stone pad approximately 3" to 4" deep and extending 1' on each side of the utility shed or gazebo. 

Also, pressure treated 6" x 6" 40 CCA timbers could be installed to help retain the stone.  The top of the timbers should be installed 1" to 2" above the existing grade.