For those of you that may be considering this I found a couple more battery options.
There is a Miata replacement battery sold by AutoZone, Duralast Gold p/n 8AMU1R (AGM type? approx 5 x 7 x 6.5 @ 25 lbs) this sells for $84 and is approximately the same size as the interstate battery shown in the pics. It has a 3 yr full replacement policy and 8 yr warranty. There is also a WestCo replacement battery for the Miata which costs more and claims a 425cca vs the 330cca for the Duralast, however I contacted WestCo an was told that it was 320cca @ 0 deg. I had read several places that all the replacement batteries from WestCo and AutoZone are made by the same vendor and that pretty much confirms it. AutoZone is cheaper and easy to service under warranty. These batteries are a standard trunk install in the Miata.
While I was at AutoZone picking up my new battery I noticed that they had some tractor batteries on the floor. In fact, one with a similar look of the Interstate used with this battery tray. It measures 7.75 x 5.125 x 7.125 to top of post. AutoZone p/n U1-1 or reversed posts U1-R1. These are a bargain at $25 230cca and 1 year warranty.