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Following the usual sound post anxieties I once broke down and bought a Gemini sound post setter. It was $75 which is not much more than a visit to a luthier if I could, in fact, find one I trusted.
I messed up my first attempt but then reset the sound post on two violins without any problem ("So easy that a caveman could do it.")
One violin had been to two luthiers for adjustment and neither really did anything useful. I reset the sound post to a position far from where the "experts" suggested and the violin never sounded better.
Was it all worth $75?. No doubt if one does the necessary "homework" about sound post handling and precautions.
Note: I was just resetting posts, not making new ones which does require the other tools/gages.
When you come to a fork in the road, take it.
Y'all are making it too hard - a sharp knife (x-acto or similar) and a bit of sandpaper will shape the post just fine and size can be roughly determined by inserting something like a chopstick into the upper F hole. A bent coat hanger will install the post with minimum frustration, just be sure to tie a string onto the darned thing so you can fish it out when you knock it over while getting it set into perfect position.
I thought I might have to spend some serious money to get my soundpost in, but it turns out that necessity (and being cheap is the mother of invention!
I ended up using an upholstery needle and a bent clothes hanger, but ended up with a severely marred sound post in a slightly crooked position when all was said and done. It had so many marks from not being able to stay on the hanger when inserted. How did you manage to not stab the thing to death?
Well, I never said it didn't look a bit battered Although the majority of the marks are from the "real" post setter. At this point I'm not too worried about it - it's not perfect, nor is it in the perfect position, so it will have to come out some time and I have other post blanks to make the "real" one prettier in the end. There is a learning curve, like everything else related to the fiddle.