[B-O-B]

I would check with a local sign company if none local spend a dime or should I say dollar. I just put a dinky 2"x2"x3' gate post up & the ready mix & gate mfg said dig a hole1/3rd to 1/2 length of the post & at least 6" or better in dia. Wind is really gonna like that sail so go deep & big. I'm bettin a truck load of concrete or more. Twenty feet high is gettin up there. Like what was mentioned set it on a steel base so you can level it & do repairs.
Beautiful sign by the way.