I've started to design the first of three motor mounts:
The Honda TDI Project
As for the aero question... the diffuser on my Lotus was designed for a race application at higher speeds (100 mph+). I don't think it'll have a significant impact on the Insight although I do plan on doing a lot of driving above 75 mph with the car once it's finished. A simple under tray will probably have more effect on gas mileage with this car