Layer 3
Pillar drill
4mm bit
10mm bit
Handheld drill, 2mm bit
10mm countersink bit
Layer 3 in 15mm MDF
2 61" countersink woodscrews
4 threaded M4 blind nuts.
Time: 10m (cut + mark posns)
Cut out clearance for power socket or any other parts required.
Place layer 3 on top of layer 2 in plinth, checking clearance and fit.
Plug the left and right suspension adjuster blocks into the plinth, thread through the adjustment rods, and thread on the adjustment wedges.  Place the carbon fibre suspension hangars in position; push them up against the block and wedge, using a set-square to ensure they are parallel to the front and back of the plinth. It can help to use tiny amounts of blu-tak to fix the hangers to layer 3.
Lightly mark the front edges of the suspension hangers on layer three (from the plinth edge to the innermost fixing bolt hole at least.).  Make the pencil line as close as possible.
Use the set square to locate the back edges of the hangers, and mark carefully on layer three, near the edge of the plinth.
Mark the exact centres of the two fixing bolt holes; double-check the markings against the drawn front edge, the edge of the plinth and the measured locations of the bolt holes in the hangers.
Remove the suspension parts.
Mark two points between the centres of each fixing bolt hole pair; these are for screws fixing layer 3 onto layer 2.
Remove layer 3 from the plinth and copy the markings onto layer 4.  Tolerances for the gap are generous at the front, but should be tight at the back so that the top edge of the hangers can be braced against the cut edge of layer 4.  If too little is cut away at the back, you can always shave off more.
Pillar drill and counter-sink the two layer fixing holes.
Pillar drill the four 6mm diameter 10mm deep holes for the flangeless M4 blind nuts used for securing the suspension hangars.  Screw in the blind nuts.
Rub off the markings on layer three, and place back in the plinth.
Drill through the two fixing holes, and test-screw layer 3 onto layer 2.
Bolt the suspension hangers onto layer three (no need to fit the suspension adjustment parts).
Now you can measure and mark the location of the suspension pivot holes; the left hole is 93mm from the left inside edge of the plinth and the right suspension pivot hole is 75mm from the right inside edge of the plinth.