Use a 12" tube and a 12" adapter plate. If you want the square base to be inside the diameter of the pier (no pointy edges poking out) it needs to be 12". The diagonal of the base plate you showed is 296.98mm, and the diameter of a 12" circle is 304.8mm. Just about perfect.
My biggest issue is finding the "plate" material. I see many are using I think 0.25" thick or maybe 0.5 inch thick? aluminium, but are finding it at random places. I suppose if i cant find a source locally i should just try some ebay or amazon sources.
I think there needs to be at least 2 inches clearance from the J bolts to the edge of the 12" adapter/cement to prevent issues. So not only is this cem120 mount square, about 8.25x8.25, but now i have to have the holes beyond this area for the jbolts and still have 2" from the edge, i guess its all doable, maybe 1" over on the template make the holes for the bolts, making sure i leave 2" from the edge.. of course all this once i find the metal material
Its holding up the show on pouring concrete etc.
You can make a wooden template and drill some holes in the same hole pattern as your mount base plate. The template needs to be bigger than the sonotube so that it can rest on top of the tube. After pouring the concrete, insert your L - bolts in the desired pattern using the template as a guide. You can use a few washers as spacers at the edge of the sonotube to keep the template slightly above the concrete so it is easier to remove or make the template big enough that you can tap it off with a hammer. Be sure and smooth the top of the concrete with a trowel before lower the template for final set.
I guess i could cut a 13 or 14" disc and draw a 12" circle on it, then draw by hand the mount template in the center, then pick 4? random spots on the perimeter for the jbolts and i'd be ready to pour (leaving about 2" space between the wood and the 12" pvc pipe?)
Edited by markm75c, 14 July 2019 - 11:36 PM.