My ECU is also in the boot (trunk for western hemispherical challenged people) - but
I have Alucraft on the engine-side of my boot and since that was installed, I can hardly feel any temperature-rise in there at all. It makes a world of difference.
Hmmm... My old Citroen zx had the engine management box fitted in the enginebay.. by the factory.
Maybe its because this new one is an "after market" or "tunings" ECU that its supposedly more fragile..
Either way, this summer I noticed that it gets very hot in the boot of my car, so Im not inclined to gamble that it wont fry the electronics if I fit it there