[streetpro]

I got a china knock-off diffuser that is made of fragile PP plastic so once slight touch with anything was enough to break off 3 clips and one fixing point.
It was DIY! time...
1 - jack up the car, fix it properly, place a stand to avoid fall down related recovery period
2 - dig the garbage to find a carton/paper box sheet
4 - do some basic measurements, cut a rough shape a try it on
5 - ask why point 3 is missing and start looking for scissors and sharp/tool knife
6 - re-do the shaping to a fresh muck up as your 1st attempt will be off for sure
7 - transfer it to plywood, alu sheet, ABS plastic or whatever you desire to fabricate it from
8 - ad same rakes/fences to guide the air and to reinforce it (remember, idea is to have the air attached as long as possible, make it gap-less fit to a plastic end part of diffuser, fences should be going from small to tall and narrow to wide, air behind the wheels should be excluded as it is to turbulent anyway)
9 - paint it black, make sure it does not touch the exhaust pipes or does not melt if made out of plastic/fiber
10 - go for a drive and enjoy the feeling how it sucks panties under your car along your drive path

PS - I am aware that is way too small to create any downforce, its main purpose it to secure the diffuser and to cover those giant holes in between differential, exhaust and bumper...

[Wolf 335]

So what material will you be using for this?

[streetpro]

Thin plywood (cardboard is still little floppy) and then stainless steel sheet reinforced by 4-6 fences... there are 5 strong fixing points at the rear (3 diffuser, 2 bumper), 4 on the sides (bumper), 2 at the middle (heat shields points, I will create a holder with a single fixing point, most likely just a bended sheet with a slip on thread) and finally 2 points at the differential/support subframe (drill 2 holes, thread or any fitting).

[MILLER335I]

Did you ever finish this? Did it help with gas mileage?

[streetpro]

I have been busy with other things like heat-exchanger delete, ICV valve, intake ports cleaning... but this one I did to the other DD Saab 9-5, there was a huge hole before, no cover what so ever... and it made an effect, quieting the front, possibly better Cdx :-)