I think the best options so far are:
1. Fusebox on top of ECU: (3 dimensional thinking)
2. ECU at right angles:
3. (not pictured) , ECU as per option 1, Fusebox next to the lower button panel
I've been thinking about all sorts of combinations - still not completely decided. The ECU bottom edge must face down to provide access to the connectors, the fusebox might go on the other side of the heater which would be even better - out of the way of the heater hoses.
There is room for the ECU on the driver's side, but might be limited by the loom....
Still not completely decided - I'll add some more fixed parts and hopefully it will all become clear.