You can do two passes using an inch pound wrench. Crush the rubber seals in the first pass using 60% of the value (if it's 21 in.lbs, use 13 in.lbs), and then the second pass use the recommended torque spec (21?). Start from the center of cam cover and then outward in a clockwise motion. Center top, center bottom, top left, top right, bottom right, bottom left. The day after driving it when the engine is cold, do another pass at recommended spec in the same sequence you started.
EDIT: Make sure the threads are clean. Wipe them down, use carb cleaner too. Squirt in the pocket, squirt the threads on the fastener too.