Currently debating whether to buy a spare and bigger battery from Specialized at a very good price, or do a DIY one using a Chinese bottle battery and the instructions above, plus a new loom, and saving a few hundred quid. But as I've not had time to get the current battery discharged yet, it's...
Presumably you could add the diodes between the output of one of these pre-made bottle batteries and the motor (i.e. inline with the wiring loom?)
EDIT: seems this is essentially what you did with your battery pack anyway! So it should be easy to do.
However, the only problem I see remaining...