Here - I managed zero excess heat: http://www.talonfiremage.pwp.b…n622314zzk5hnt83hkiyn2hhc
How? I'm not even sure myself, but for some strange reason putting a heat capacity plate in the remaining empty slot minimally improved overall heat distribution, and with some component reshuffling I ended up getting 635 out of 640 cooling utilized. Even the exact type of plate seems to matter (it doesn't work with a normal plate, and only half as well with a containment plate). I have no idea what's going on.
Unfortunately that doesn't keep that one vent at the bottom from melting after four minutes, and every attempt I make to move heat away from it ends up reducing cooling by a way too large amount.
I also had slightly different heat distribution results when I moved the location of the fuel cells to a different corner, but I didn't manage to make anything intrinsically better out of it.