I was concerned by vibration issues and wanted to isolate the M800. I also didn't want to risk voiding the warranty by removing screws from the ECU (probably wouldn't but didn't want to risk it). Mounted an aluminum plate to the car chassis using rubber vibration isolators. I got mine from Mcmaster Carr (
http://www.mcmaster.com/#rubber-isolators/=4d7daz). Once the plate was isolated I wanted to attach the ECU to it. Used some aluminum angles and small 1/8 countersink aluminum rivets and adhesive backed rubber sheet. One of the 4 angles has a relief cut from it for the M800's plugs. One leg of the angle was riveted to the plate and the other had the rubber applied to it. If you pre-drill and layout the angles you can get a tight fit on the ECU body. To make it a little tighter you can slightly bend the outstanding leg inwards. I may add a tie-wrap but it is very tightly secured this way regardless.
Packaging things like this made it possible to mount an ignition expander and traction control unit on the other side of the plate (well within 500mm wire run limit). You can see the wiring to the the units in the first picture.