I ordered the Stillen diffusers and from all the descriptions it looked like they were singles and fit either side so I ordered two. It turns out that it is a pair so I have two full sets.
Stillen would charge me a restocking fee and return shipping to return one unopened box, so I'd offer to send the extra here to anyone for the Stillen price which is roughly $325 for the pair with shipping included.
These are the standard OEM sized diffusers (Stillen also has an extended length). The kit includes instructions and replacement rivets and the units include divets where to drill out to match up to the holes in the belly pan.
I assume that everyone is familiar with the diffusers and that they individual parts cannot be ordered from Ford, and that the entire belly pan has to be replaced at a cost of $1000+. The Stillen replacements are made from a more forgiving material although still pretty stout.
Anyone interested PM me. I have a couple of weeks before I have to return them to Stillen.