Seats from Vintage Motor Car in Hawaiian Gardens fit - they make a thinner speedster seat (shown $605 ea) and a more comfort cabriolet style.
Porsche 914 seats are comfortable and will fit - they do tilt back with the OEM 914 base.
What Miata seats do you have? - the NC (last produced 2015) was the largest inside. NA and NB/ND (maybe) were smallest. Would making a lowered seat pan get them low enough for you? Like a metal paint paint roller tray added to floor pan that gives new 3" more.