A cooktop that is rated 208V, and can be wired for 1 phase? That sounds like a strange animal. You might want to double check how it is actually wired just in case it can be adapted. If wired for single phase, what would its voltage requirement be, then?
I have seen some cooktops that say they work on either 208V or 240V, and list the same kW rating either way. I'm not sure how they accomplish that, but I suspect they may be running each element on 120 volts and don't really care what the phase angle is between the phases.
capt al: do you have a wiring diagram for 240V 4-wire delta (the customer probably does not have 240Y/139) via buck/boost to get 208V three phase? Or were you suggesting doing this only in single phase?