I would suggest the easiest solution would be to install 4 corner supports (8"x 8" concrete pyramid block) that have been dug into the ground enough to be sitting flat on a surface area that is 50% larger than the block measures (this leaves room to achieve square). Attach 4"x 4" post to blocks...
My partner and I are into our third year of homesteading off the grid. We aren't religious but we consider Country Wisdom and Know-How to be our so called bible. This book is incredibly informative with easily applicable instructions on just about anything you could imagine needing whilst living...