A soundpost is a wooden support that fits between the front and back of a string instrument like a cello. The exact placement of it is a mysterious art involving much trial and error.
However, because the front and back are not parallel surfaces, you have to create a new sound post of a different length if you want your soundpost in a different spot. Matching the slope of the interior surfaces is also difficult.
This is my vision of an adjustable length sound post that could be put anywhere. The center is a "jack screw" which has opposing threads on each side. Turning it would push or pull the two wooden parts.
The ball joints at the ends are held together by magnetism.
Unfortunately, about a day after I thought of this someone announced a commercial product that does much the same thing... for 500 euros!
But maybe I'll make this for myself. I figure it's about $15 in parts and postage.