Variac design question

After viewing Mark Finnis' 3 phase variac at
I was wondering how your construct something like that with just 1
phase. It looks like in the pics that there are 3 separate cores.
Could you possibly make a tube shaped core and wrap it with magnet
wire. It would basically just be an iron core inductor. Setting up a
way to tap it wouldn't be that hard after putting it in a bracket of
some kind. How'd you make a core like that? Possibly make a square
core and cut it to make it round, or stack round laminations up to
get the length. It seems like this would be a much easier way to
construct a variac opposed to a toroid core. The closest I've come
to making a transformer is using 2 MOT's to make a 120v isolation
xformer. My 140 20A powerstat does the job for now, but I'd like to
make my own custom 280v variac in the future.