I've seen tons, and sorry to IT7 guys,(I used to be one of them) most of the homemade stuff looks, and performs like crap.
Making your own airdam is time consuming and likely more expensive than buying one.
Mike at ISC has a good one that fits well, is legal, and functions properly. And it's cheap. problem is, shipping is pricey and he might no longer be making them, as the 1st gen Rx-7 is nearing the end of it's run.
Call him and see if you can talk him into running a batch. Then get TWO. The shipping will be halved essentially, and you WILL need a second, unless you have it as a spare.
If Mikes not cooperative, then try and find someone that has one, get the proper mold release and resins, cloth, etc and pull a mold off it. it's a good piece, and could form the basis of something even better.
I know Dan Sheppard up in NH has one on his car since that one used to be mine. He'd probably be willing to talk about molds and such, but I think before you do that, call Mike at ISC.