I got my CEM60 adapter plate finished and working last night.
I think I bought my aluminum plate from the same seller as Whuppy. For future folks who want to do this. 6.125"x3/4" is near about perfect in size. I think 6"x1/2" is the smallest I'd go on the dimensions for the round plate.
Also note that the spacing for the two threaded posts is 130mm. On the iOptron blueprint, there are two sets of holes, one set at 120mm and one at 130mm. See those extra holes in my plate?.... :-)
I ended up using those extra holes to bolt the plate to the bottom of my disc brake mounting bracket that anchors to concrete in my pier. I just drilled 2 holes into the top of the disc brake using the 120mm holes already drilled in my top plate as a jig and used two M8x1.25 bolts to hold it down. It's very secure.
For the center post, I used MR126ZZ bearings held in place with a M6 cap head screw. I threaded a hole in the center of the plate to accept the M6 screw to hold the bearing down. The height dimension isn't critical as the CEM60 has an open hole in it's bottom. I stacked 4 bearings making it higher than the recommended 15mm, but it's no problem.
The bearings have a 12mm outside diameter and a 6mm inside diameter so they fit perfectly into the bottom of the CEM60, and 6mm bolt holds them in place "well enough".
I don't think the smooth action of the bearings plays a large role, but this thing was so incredibly smooth to polar align last night... it just went exactly where I wanted it to go. Such a dramatic change from my Atlas mount.