Thanks once againfechter wrote: There are lots of ways to do it. If you're just building one, I would just lay copper wire on top of the high current traces and solder it down.
In factory made controllers, I've seen a variety of approaches. One is to make a copper bus bar with legs. Legs are located close to the FET connections so the PCB traces are short and can be bridged with solder. The bar goes on the top of the board. You could do this with heavy copper wire as well.
Another approach is a strip of copper sheet that is cut to the same shape as the board trace and laid down on top of it before soldering. Much like a wire, but nice and neat. These traces should be extra wide wherever possible, like 0.25".
You can get an idea from the picture below. I think they just piled up a bunch of solder, but some copper in there is much better. The actual connection to the FET leg is narrow but as short as possible.
This is what I've done. I have managed to place 3, 70 thou traces between the transistors. 2 bottom copper and 1 top copper. I think these should be sufficient for 20A however I will also add some copper wire.
What do you think?