You need two bikes in the trunk of a Miata? I can fit two bikes in the trunk of almost every standard car. However I have been unable to even fit one folder in the back of many convertible's. I never tried with a Miata, but I think you may want to get a trailer hitch and a hitch mounted rack....or look at
https://www.seasucker.com . Otherwise your restrictions may leave you with zero options.
Thanks,
Yan