baseboard plan
Check space in plinth for size, remembering the plinth may not be exactly square.  If not, find a square corner, and work out the shape from there.
The size should be around 418 327mm.
Sanding block
Pillar drill
Electric drill
44mm, 64mm holesaws
418 327mm  6mm MDF, 5mm acrylic or rigid foam-core Carbon fibre.
Time: 1 hour
Cut the baseplate to plinth size.  Check fit and sand to size if necessary.
Drill the three holes indicated in the baseboard.
Right is an exit hole for tonearm cables whose plugs are too large to go through the original hole in the back of the plinth.
Centre is a clearance hole for the long motor/tacho unit, which protrudes below the subchassis.
Left is a clearance hole for the spindle base. This avoids the subchassis hitting the baseboard when it is bounced.
If material is more than 5mm thick, you may chamfer the bottom edges (if appropriate to the material).