Tostino, The copper is a nice idea!
This is great to see we are at least two persons here who want to test these 36 FET beast!
I would recommand you to ensure that the mosfet are not too thightened because it is know that the TO-220 tab bend easy and make small deformation that lift a bit the area right under the mosfet where the silicium chip is located. This area is exactly where the heat os the most important.
Using screw on the To-220 is more technical than it appear. The copper thread you have must not deform the flatness of the copper bar when you thighten the screw. Just few micron of space will destroy your good thermal resistance between the fet tab and the copper flat bar. Copper is soft metal and on 1/8 thick it can deform easy the faltness under the FET. Often you can't see it with eyes but there is real gap
That is why i think i blown many controllers.. there is only need of ONE FEt incorectly thightened to weak the entire controller
This time i prefered applying pressure on the mosfet instead of using screw. we'll see..
If you use Kapton, use very small qty of thermal paste to fill the little air gap of the imprefect flatness of the surface. Kapton is not soft enough to fill these gap.
Here is the link i use as a great information referencehttp://sound.westhost.com/heatsink3.gif
I look forward for your results