I dont have time at the mo to go about designing some finned ones because of other work I have on, will get onto it as soon as I can, but i have made some flat style ones which i think i will run with for the time being. A few self tapping screws should suffice to fixing as they wont be seen underneath.
Anyone interested in a set just ask.
Here's a few pics of them on my car (attached with masking tape for picture taking)
A set of these diffusers will be £35 + postage (prob £5 or £6 in UK) Dont know what shipping would be to US though, I could get a set packaged up and if you give me your US address I could then get a price.
I can make these diffusers in black or white (your choice) but will need colourmatching/painting by yourself.
I am going to fix mine on with a couple of self tapping screws into the underside of the bumper which would make them no problem to remove if need be.
Other method of fixing is by using automotive adhesive such as Tiger seal or sikaflex, once stuck with this stuff, they are stuck.