I have a 50x60x8mm copper plate for under fets and 40x40x3mm for upper fets (from processor heatsinks) which will be attached with 4 screws after milling the larger plate to get them together and enough room for pcb/fets.

I'm happy with the under plate but wondering if the smaller one can transfer heat well enough to the other one due to quite a small common area 10x40mm. I don't have larger one, so I have to use this. Let's see if it's good...

You gave me great info GriffinRU, BrianG and Zeropointbug. I wouldn't have even tried without them!