I was curious if there was a good way to fill the knot holes in cedar siding. I have a clear finish on the siding, so any "plug" would have to match or look natural. I've heard of using a tapered rod, but I imagine that would be difficult to match up.
I once had a house with stained rough cedar siding, and the woodpeckers had done a real number in it. I ended up filling the holes with Bondo--which of course looked NOTHING like stained cedar siding--but once the Bondo was painted brown you couldn't tell it from the original.
It may not be the most elegant solution in the world, but it worked for me.