why dont you build it on a base. It doesn't have to be that wide. Use 3/4 ply, trim it out and add some ground habitat. secure the posts from underneath with long lags and use steel corner braces to help support it. You can mortise out a blind slot in the bottom of the posts and epoxy the L braces in. When it sits on top of the ply, screw it down. Add dirt and leaves to cover. It'll look like you dug up the whole section of ground. If you need a pic, let me know. I just fastened a fence post that way on a piece I just finished
I need it to be portable and have easy set up and take down. Its a display that I'm using to lease to banquets and stores, so portability and handling ease are important.