First, let me start by saying that I only compared the OEM Mazda thermostat with the NAPA sourced thermostats. The Miata uses a generic, common sized thermostat with a 52mm / 2 1/16" diameter. NAPA was the only parts store that would give me a hard time when I ask for a generic 2 1/16" thermostat in 180* or 160*. Every other part store would only look up the Miata and sell me whatever the stupid system said was the one. Just walk into NAPA and ask for a generic thermostat in 2 1/16" and 180* temp with the part/reference number of 142 and they should be willing to help you.
Here is a pic comparing the OEM thermostat with the 180* #142 Napa thermostat. The NAPA is shorter but it makes no difference as it fits the thermostat housing / water outlet housing perfectly.
Here is a pic [attempting to] measuring the size of the hole that the water circulates through when the thermostat opens in NAPA vs OE. I couldn't find my calipers so I used a tape measure - sorry. The diameter of the hole is very similar. Perhaps only a mm larger in the NAPA. Napa on the left. OE on the right.