After a break of a few days, I finally got around to completing my Mendel90 print bed assembly.
As mentioned, I'd agreed with nophead to accept some parts that were not finished, and the only task that needed doing here was tapping the Dibond to M3.
To do this I used a tapping chuck:
This is ideal for tapping soft, thin materials as you can turn it with one hand, whilst using the other to keep the tap straight.
I was then able to fit the M3 spacers, and after wiring, attach the heater PCB to the carriage: