A contour gauge simply works by pressing the firm – but free moving – pins against the object you want to take shape of. You now have an accurate template for tracing the shape on the piece you need to cut and fit: rounded post, trim, or whatever complex object / shape that needs working around.
The next step will require a jigsaw, bandsaw, or coping hand saw for the cut.
Where to Find a Contour Gauge
I’ve been using the 6″ model for several years now, and can’t say I have any complaints. For purchase: Ace Hardware, True Value, Lowe’s, and DoitBest carry this item in stock, so it’s readily available as well.