I printed a 3 part adaptor to mount the Mitisubishi FTO speedometer inside the mini instrument cluster. I printed them in PET-G which hopefully won't soften in the sun -- it's apparently good for 75°C so that does seem unlikely. I must thank Daniel Dillan at www.vivenda.co.nz for suggesting PET-G and advice on getting good results with it (print it hotter and slower than PLA).
I'm not entirely sure why I needed so many prototypes. Good thing I have my own printer.
I now need to create an appropriate speed pulse signal from the inverter. Initial investigation suggests I cannot configure it to make less than 6000 pulses per kilometre, which is a little more than double what the speedometer expects. A simple arduino sketch will fix that.