Bernd,
The machine was built over 10 years ago using the plans in the book "Electrostatics" by A. D. Moore. Perhaps a few pictures and hints that I can remember will be useful.
The hub was made from 3/4 inch acrylic and the rods are from 3/8 inch aluminum. Shallow grooves were turned near the hub end of each rod to help anchor them in the hub using epoxy. Vent holes were drilled to allow air and excess glue to escape.

I made the frame from polycarbonate because I had some from another project. Stainless screws were used at all joints.
Some more pics:
Charge collectors

Inductors

The machine needs conductive brushes, I used material from static protection bags used for sensitive electronic devices years ago. These are the kind with a black opaque coating, never throw anything useful away!. I'm not sure if the more modern film coated plastic would work. Wire brushes could work but might scratch the parts.
Stew
Van de graaff generator

home made one

15 inch dome with 12 inch ruler

The copper band was formed in three sections using soft hammers to beat the annealed copper sheet into a spherical aluminum form. Much work! The copper was from 4 inch pipe purchased from a scrap yard before the price of copper spiked. The stainless bowls had to be hand worked because they are so thin. This project is on hold for the moment awaiting ambition.
Dave