Watch out for skin oils left on the copper from the hands or fingers. Use some gloves or plastic when holding the WB. Brasso and any metal polish is good and use alcohol/acetone or TIM cleaner to clean off the leftover polish.
Cold Plate thickness at 3/8" is best compromise between performance and cost. Still 1/8" or 1/4" is good enough.
http://overclockers.com/articles305/