Layer 4
4 Layers
Pillar drill
4mm bit
Handheld drill, 2mm bit
10mm countersink bit
Mitre saw
Layer 4 in 15mm MDF
6 61" countersink woodscrews

Time: 1.5 hours
Carefully cut the frontmost lines with a mitre saw.
Test fit the frontmost piece of layer 4 in the plinth on top of layer 3.  Shave off more MDF from the back edges if necessary.  When happy with fit, fix temporarily with double-sided tape, or some glue.
Carefully cut the back lines on the mitre saw; better cut away too little than to much, as you can shave off more if necessary.  Check for fit on top of layer 3.
Cut the channel for the motor wire, either to the  internal speed controller board or to the external speed controller socket.
Fit the remaining parts of layer 4 with double-sided tape or glue.
Remove the suspension hangers, and unscrew layer 3 from layer 2.
Remove layers 3 and 4 from the plinth, refit the suspension hangers, carefully turn over and drill through holes for screwing them together.
Remove double-sided tape if used, and screw the layers together.
Place layers 3 and 4 back in plinth, screw in place and refit suspension.
Check clearances and operation of suspension adjusters.