The battery from Autozone here isn't an AGM battery, but the O'Reilly one is. Both AZ and Wal-Mart both only had regular acid batteries. I'll take a pick tomorrow of the custom aluminum tray that I made.
Here is mine with the tray that I made. I picked up a small piece of aluminum, made a template, then bent the aluminum in a vice. I heated it a little to make sure that it didn't crack, but it didn't break in my tests without heat. I picked up some battery holds at AZ for about $3 and the top bracket for about $5. I powder coated it at work. I picked up a sheet of a rubberized foam with an adhesive back for $1 at Hobby Lobby to guarantee it wouldn't slide at all. There is also a bracket that bolts to the transmission mount bolts and attaches to the tray for extra support. I've got about $15 into this tray. The UIP in the picture is an old ETS pipe, but my Buschur short route UIP clears it without a problem. I will take pics of it when I finish breaking in my new engine and go speed density which will be about 500 more miles.
I'm looking into this as well. I would of thought the CCA of a miata bat. was over 400... but almost all of them state 370. There are some better specs on some of the braille batteries... although price is ~$50-75 more.
For what its worth, it was around 32* here Friday and my VIII started strong on a duralast gold miata battery from autozone. I'm sure the higher cca the better but how much does one actually "need" .
Just general information. Most Evo replacement batteries are 500+ CCA and a lot are in the 600's, so apparently Mits thinks that's what you need, however, they also design the vehicles to survive (like most OEM's) in -40F weather, so if you lived in Minnesota or Alaska ... where ever, it certainly is 0F for a lot of the winters, -20F is common for mornings.
Its not necessarily one morning or day it was 32F, its for constant time periods which will actually start bringing the internal temperature of the battery down which will make it hard to start, these are the reasons why you need high CCA.
On a second note, your battery is an energy storage bank, if the OEM didn't believe it needed to be the size that it is, than all batteries would be tiny, its not like OEM wants to add weight to your car for no reason...but again it depends on the usage and the design requirements the manufacture believes you need.