If you want a really solid pier that is as solid as a rock go for a 250mm or 300mm sewerage polly pipe cut to the required height you want, dig your 600mm cubed hole, place your polly pipe in the hole, half way up the base cube, get it straight and fill the whole lot with cement, then when your cement is still wet stick in you base plate bolts, attached to a square piece of particle board the same size as you base plate, and let it cure for a week or so and it will never move.
Leon
