I've got a steep slope that needs 12 inch concrete gravel boards, and
I need 6ft high panels.

The local DIY superstore only stocks 8ft long concrete slotted posts.

That would only leave 1ft in the ground ???

Is this in order, bearing in mind that the post would be concreted in.

Or would I have to dig the hole 2 ft deep and fill in the bottom 1ft
of the hole with concrete. Then post on top, infilled with concrete.
Or would that not be to secure ???

How does everyone else do it ???

TIA