Just a total guess but I would stack up sizes like 8; 9 or 10, then another 8 and carve it into the rounded shape.
OR: stack 3 of the the same size rounds - like 10"ers, put a smaller cake board (8" or 9") centered on top of one end and carve around/down to the center; flip and do the same again - just saw something similar on t.v. :)
i've done those -- i just carved it out of foam -- these people have all the tools -- but i've carved with steak knives too -- a big fluted tier -- i just loved that thing --
https://hotwirefoamfactory.com/
the big word scared me -- had no idea that shape was that word -- ha!
steak knives and sand paper -- you gotta go outside -- the mess is quite messy -- i have a lovely back yard and am blessed that way --
but if you have the cutting tools you can do it all inside except you need to keep fresh air going
well except for the sanding -- you still go outside for sanding usually --
i think i threw one of those toroidies away recently -- oops 
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