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!

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.