You shouldn't need a piston lock or any other device for keeping the engine from turning while tightening the nut. These are needed for helis where you have notheing decent to hold when tightening the nut on the clutch/fan assembley. If you grab the fan wrong you knock the fins off of it and have to get a new fan. You are not going to have that problem with the prop.

NEVER stick something n the exhaust port to prevent the engine from turning while tightening or loosening the prop nut. That is a good way to break a piston ring or to put a dent inthe ede of the piston... this will result in destruction of the engine.