Disclaimer: I've never owned a Miata. That said, I have owned several convertibles in my life. None of them sealed as tight as a sedan. As Black Lotus said, you just really need to get out there and drive it. If you still smell fumes and you're not sure where they're coming from - and - you have a trusted mechanic, take it there and see if they can find any leaks from your car.
In my experience, driving a small open sports car on the freeway just isn't fun. I try to stick to scenic two-lanes when I'm driving my Triumph.
SL