I have a Yakima trunk rack that I bought for a Honda Civic, but it's also been used on my Protege5, my wife's Protege sedan and my son's Tercel. It's almost infinitely adjustable, and I'm sure it would work on a Miata (I had a similar one on a Fiat 124 30 years ago). Can't remember the model, but if you search the Yakima site I'm sure you'll find it. Mine cost something like $110 at REI.