Personally, I've never bothered with balancing the props except for checking them before and after each flight for any nicks or bug splatter. I guess a piece of 3mm rod that is straight and free from any surface crud would work (P4). I would be inclined to use a dead flat friction free surface where the rod is retained in place when you spin it, having visions of the the rod spinning off the glass. Had a look on fleabay and I would probably just buy one of the Dubro kits and be done with it. It can sit there with my two chainsaws, pole saw, brush cutter and electric chainsaw sharpener and a host of other stuff that pays for itself each time I use them but for the most part just stare at me in the garage when I walk past them.