My final decision on the fusebox / ECU position; The heater is going to be where a stock Zero fusebox goes, the Fusebox will be next to the wiper motor & the ECU sit in front of the wiper motor on a custom bracket:
1x envelope sketch & 2x cereal packets later I have a template I am happy with, most of the re-work spent simplifying it and making a decision over mounting to the scuttle or the chassis. This bracket will mount to the chassis and the scuttle drop down over the top:
I used 3x metal cutting disks on the dremel to cut this out of 1.2mm stainless. I have no metal bender but all things considered it came out pretty much as planned:
The wood is to mark 100mm in front of the firewall to make sure I don't use the space for the wiper motor:
Drilled; 2x rivets into the side chassis tube & the tab on the diagonal chassis tube will be bonded in place (possibly riveted too once the bond is dry). The holes along the bottom allow access to the ECU programming plugs and status light:
Mocked up just before fixing to double check positioning & make sure I still have access to scuttle/windscreen mounting holes. I'll play the stability by ear, if it wobbles too much I can run a stay back to the firewall easily enough:
I was mulling over a false firewall inside the scuttle to mount the heater & fusebox; that might be overkill though...