The baseplate of a Hammond Multiplex typewriter is actually part of the machine - it does not work without the clearances and spacing of the baseplate. As seems to be very common for Multiplex typewriters, the glue used to assemble the base (and lid) lost its power after a century and the thing starts to warp and fall apart.
To repair the baseplate, all the planks that showed even a little movement were completely taken off the central section. The top layer of thin plywood was then first glued down firmly to the central section. All edges of the planks and central section were sanded back for a clean fit when re-assembling. This sanding back also allowed to compensate a bit for the un-even shrinking/expanding of some planks. Glueing with regular PVA, firmly clamped and left to harden overnight - one plank at a time.
The original felt on the base was completely removed and a new, thin sheet of felt was cut to size. White pencil to trace the outline and the holes, then scissor and hole-punch.