Airdam Attachment method
 

My airdam is a piece of lawn trim like everyone else seems to using. Bottom side is a cylinder for strength, the upper side is a design with 2 ridges on one side and a 3rd on the back. All of them are short (0.75 cm) and flat, not ideal for much. I cut mine down to only one ridge facing forward and used that to seat itself around the inside of the lower lip of my corolla's front bumper. this lip is open for most of the bumper and closes towards the wheel. I then drilled 5x m6 bolt holes through the plastic bumper, placed a small L bracket above the lawn edging, placed my aluminum flashing chin pan over that and fed the bolt through for a washer and nut to hold the whole thing in place. The finished look is clean and if I chose so I can cover the bolts with a plastic cap, but they're easily out of sight. Right now the airdam is sturdy enough being held forward with the L bracket, but another set of bolts could be put through. Flat T brackets would be ideal for keeping shape but I would rather not because scraping would likely bend it or put more pressure on the above assembly and eventually break the bumper. I'd rather just replace the 7$ lawn trim when it's done.
The dam is terminated before the wheels where the body is screwed together. An L bracket is put in to give me a mount for the side of the dam. It keeps the whole thing taught along the bumper lip. It's not the perfect place to mount but a few spacing washers make it work. It also gives me a place to put a zap strap mount for part of my bellypan.

That sounds very thorough and I think it will certainly hold shape in motion and flex when it nails stuff like speed bumps. Mine is MUCH less aggressive in attachment. I drilled holes about as you have, but without brakets of any kind. I used screws for most of the holes, except one, in front of the front tow hook, which is on the driver side. On that end mounting point I used two strips of plastic insulated wire twist tie (so I can remove it easily in a towing situation). Not ideal, but convenient and it works. Yours is better, but harder to reverse on demand.

Looks good to me.

I was wondering how I might make it easier to take off if I need to. Wingnuts would be ideal and hide inside the bumper assembly. I've kept all my bolts as m6 10mm because so much of the body and engine uses that size.

If you don't mind, I would love to see pictures, as I am getting ready to put on an airdam of my own.

Pictures of the mounting locations/methods you describe would be very much appreciated.