The fixing is with 4 tabs riveted (floor rivets) to the flood rear edge made from 25mm strip. The diffuser slots in above these and held with buttom head screws through the tabs into rivnuts in the diffuser. There's 2 rivnuts in the back coner of the diff carrier and at the rear skirt I have (7?) ally tubes to space the diffuser from the skirt. I've used some plastic threaded bungs from B+Q in them with screws from each end. Once up it's surprisingly ridgid. I have 2 NACA ducts in the rear of the tunnel cover to feed cool air to the diff. The gap between skirt and diffuser allows the air coming down the tunnel to escape.
I did some reading before making it. There are max angles that a diffuser works at before it actually creates lift. Ideal was 15' i seem to remember. 20' max (was 2 years ago so don't hold me to those numbers). Ideally it'd stick out 2" beyond the rear panel, but I didn't want to hit my shins on it every time I walked past. The new one will be 1m all the way and I'll just let the droop flex it down. I didn't like this idea but I've seen it done and it's not a lot of movement and only at full droop.
This is why it's being re-made...
The underside is raw ally. The edges have edge trim just in case I kick it. Top face... carbon sticky back plastic.
Does it work? 
I don't know. It firms up the flappy rear skirt and in crappy weather the rear panel doesn't get full of crap... so maybe it does.adrian