Now's my chance to brag about my dear dear DH!
He made me two full sets - round and square, each starting at 18" up to 8" in 2" increments. Yessssss...I AM a spoiled lucky lady!
They are made with plywood. We tested using 1/2 foam core, but even with "washers" made from CDs, the foam core crushed when you screwed the fittings in tight enough to keep the boards from wiggling.
The fittings are 1 1/2" PVC fittings, with the straight pieces cut to about 4 1/4". The bottom has a male end going through the base with a small screw cap to hold. The big 18" base has cabinet knobs as feet a-la-Martha Stewart, but we didn't use that big a base for the pictures so I used some rubber stick on feet we found at Lowes....actually I think they were door stops.
Since we started this, I've seen simpler methods, but you won't get anything more flexible or sturdier than these!
My biggest issue after the fact is that we painted them so there wasn't raw plywood, but when we went to use the sticky shelf paper, it wouldn't stay! I had to use 2-sided tape to hold it. Any ideas?
We certainly had fun working on these together...hope it helps.
Edited to add: This one with (I think 12-10-8") held almost four dozen full size cuppies + the figure.