Armboard #2.
above, schematic for armboard #2. Thanks to Brian Kearn who has sent in this drawing with dimensions taken from an armboard produced by SME for this very application.
Construction
Multiple laminates of hobby grade Birch plywood glued together with construction cement except for the top layer which is adhered with four wood screws. If this works to satisfaction, the same layout and build-up technique will be used with better, more suitable grades of materials.
2) 
above photo #2: A final check with trammel for the required pivot to spindle distance. 215.4mm. Even though the mounting distance specification is the same for both the SME and Thorens arms, the mounting plate of the SME tonearm will require a more outward and forward location. So we simply swing the trammel a little further down toward the front. The pivot bearing of the SME arm still fits within the confines of the unmodified Thorens subchassis.
3) 
click on thumbnail to view full size photo.
above photo #3: Another trammel photo.
This photo also shows the thinking behind the layered build-up. The lower portion fits within the armboard aperture. The top plate that mounts the tonearm rides above the turntable plinth. The fasteners holding the armboard to the subchassis are below the upper armboard surface which mounts the arm. This upper surface is fixed by way of four wood screws that anchor into the lower layers of this armboard.
4) 
above photo 4: Another trammel check this time with the top plate mounted. The top plate, riding above everything, removes from view the port opening in the motor plate. With the tonearm mounted, it would take very close examination to see the nature of the offset.
5) 
above photo 5: Another trammel shot that shows the location of the tonearm pivot. Paper disk is same diameter as the body of the bearing pivot in the SME.
6) 
above photo 6: With tonearm mounted.
7) 
above photo 7: side view shows amount of suspension excursion possible with this mounting. It will be enough. One detail which required attention was the length of the mounting screws for the SME. I used #2 x 3/8 brass, slotted, round head wood screws.