ok, i just translated heat flux into german
"Hitzefluß" sounds understandable to me.
The WPS uses a single FET. The heatflux and heat distribution allover the die is no problem, because the FET is not the die, it is only the heatsource. If you don't mean the room aspect of heatflux but the time aspect, i can only say, that as far as i know a FET has a very even heat dissipation which does not change time by time. As i said the die is not included in this projekt, so you can use Diesims from Copper, steel, Nickel, wood
or whatever you want. The thicker the diesim is, the better the heat distribution allover the diesurface will be. Of course the diesim adds another isolating Layer, but i think when you don't compare results with other testbeds and other dies it doesn't matter.