Here’s what I did 600mm square hole approx 800mm deep , all depends started to hit water table , 4x 1200 mm rebar hammered into hole base to give concrete extra hold , run some 75mm gutter pipe(Bigger the better I used 40mm too small ) Into hole for cabling running to centre even if your not feeding cable just future proof , I built a frame think of it as a top hat with a lid that can be screwed to top of frame and drill pier fixing holes attach threaded bar 600mm long securing with nut either side of ply top board Remember to bend last 150mm so bar can’t unthread from concrete place in hole making sure it doesn’t snag anything, no point drilling that’s extra work and not required , hire electric mixer from hire centre ,pour concrete then push top hat into concrete and screw down and let set , you can now use the nuts on the underside to level pier if required as the concrete will shrink slightly enough to get a spanner on pics attached to give you an idea how I did it .