You don't need to design anything elaborate. Cooling fins, which simply add surface area for air to come into contact with are only necessary if the sink is only using air to dissipate heat. My chosen method is to just use a piece of 1/4" aluminum plate as a heatsink to act as temporary storage to quickly draw heat away from the esc, but have this sink channel the heat to the chassis. While I was running my Quark with this setup, the esc never got over 150. I went this route because I detest fans and will only use them when absolutely necessary.