Digital Panoramafotography - Segment-technique - Bar for Nodalpoint

For the determination or setting of the Nodal point (better said the "optical centre" of the camera), are usually some efforts necessary. One needs 2 adjustable levels to be able to adjust the optimal point of rotation. It is many products at the market (Kaidan ipix etc.), which are too expensive for me! I must build an own construction for me. A variant for approx. 10 $ which fulfils the purpose very well.
You can finds very good tips under for the determination of the Nodal point and ones's economical alternative panorama-bar.
How could I forget it? TBK also offers a very good description.