Summary

Here is a solution for my prototype SMD PCB production.
A holder is used to fix the PCB. The smd paste stencil could be fixed at the right position using magnets. Then the paste could be applied.
Very easy to use, much better than my former way using tape.
... and its possible to work with double sided PCBs.
These files are made for a 100*40mm PCB, the OpenSCAD file can be used to generate files for bigger PCBs.
Made some changes for using the (...sadly bad...) thingiverse customizer. If you have problems use the scad file in OpenSCAD.