So I had this idea. Why not try and create an underbody spoiler? Not a diffuser, the purpose of this piece would be to provide additional downforce, not just allow airflow. I'm sure this has been done before, but I've not seen it on a miata. I think using this in conjunction with a trunk lid spoiler and a front splitter would provide some awesome downforce. I mocked one up, I would probably use fiberglass or thicker aluminum with supports running the length of the sheet. Sorry for the bad pics, cell phone camera FML..
Basically make a sheet go from here:

to the top of the trailer hitch (have to keep the hitch for the tire tail)
This is what I think it would be shaped like:

It would fit in like this:

I would need to cut a section out of the rear bumper like so, you can kinda see the line I think would work (I drew it in the dirt, sorry the car is super dirty)

What do you guys think? I was originally just going to make a diffuser, but it would be kinda pointless with the trailer hitch I think.

 

While watching the Singapore night race, I heard that one of the team engineers stated 50% of a F1 cars down force comes from aerodynamics underneath the car. To back it up it always seemed that whichever driver straddled a curb and thrashed the bottom of his car wrecked a few laps later. So you could be on to something here. However I think some CFD may save you some time and headaches.

 

A diffuser if designed correctly does create down force. Are you saying you'd like to put a "wing" on the bottom of your car?

 

Basically, yes. I thought diffusers were just to increase laminar flow under the rear of the car?

 

which generates downforce, lol

essentailly when you have a lip up front, side skirts, and a diffuser youre creating a seal around the front and sides and an opening in the back, so you basically create a vacuum under the car at speed which is downforce.

 

your forgetting about all the nasty bits in the middle that need covering as well. There is a really good thread on that here somewhere, where it was even discussed whether to leave a gap in the bottom for air exiting via the transmission tunnel to escape.

 

that piece of metal under the car would do nothing without some sort of strakes? i think that's what they're called, to "guide" the air. the angle is very important, too little or too much and it's useless. the air won't follow the wing or actually, the diffuser. lastly, you could make a diffuser and have it work around the hitch. have you ever seen a new ferrari f430 scuderia rear diffuser? it looks like it was made to conceal a trailer hitch or something.

 

see if these pics come up:

http://www.flickr.com/photos/9648381@N02/3801065533/

Exige S Supercharged on Flickr - Photo Sharing!

 

BITCHIN! I found the threads I was talking about (time to fav. in chrome now)

https://www.miataturbo.net/forum/t30199/

wings and splitters? - MX-5 Miata Forum

 

something better would simply prevent any airflow from hitting the inside of the rear bumper cover, thus removing the air brake.

 

even easier is cutting out the rearmost part of the bumpercover, so it doesn't act like a sail anymore

 

The hump in the middle of the 430 (and 360 Modena) rear diffuser is where the end of the engine/transaxle sits. It's wide under the engine then tapers as seen at the back.