For a complete 2 rotor machine a possible center axial magnet holder comprised of sheet steel (2 layers ?) and plywood construction similar to the top and bottom magnet holders described above. The magnets used would be the same as used for the drive magnets, 40mm x 25mm x 10mm ferrites. The steel plate/s is used as reinforcing, and a row of magnets is attached on each side.
For the 3000mm machine, allowing 1mm gap between magnets, there are 216 magnets each side, 12 each side for each of the 18 frame sections.
For the 2500mm machine being built now, allowing 1mm gap between magnets, there are 168 magnets each side, 12 each side for each of the 14 frame sections.
Three 4mm plywood layers are added each side to strengthen the component. The first of these layers each side would be the same outer shape as the steel, and the next 2 layers would be parallel curved strips. The magnets are installed with a hot glue gun, then secured with epoxy filler around and over. There should be very little stress on this component. The main problem could be heating from eddy currents, and this could also apply to other parts of the machine.
These magnets work out perfectly in size for both machines, so this center section could be made with a quite simple template. See below for 2500 Machine template overview.