AC Delco battery replacement report. Model NS60SMF. Quality is evident.

In the US, a replacement battery, WestCo, is available for $80, the same size, but you need to stretch the battery cables. I needed a battery quick so I got the AC Delco, whose price is pretty good. My old battery refuses to start the car after only 2 nights of sitting. If I fully charge it, then turn on the headlights for 3 minutes with the engine off it's enough so that it won't start.
(The latter is a test of battery condition).

AC Delco. Same footprint but it's taller. It's maintenance free, 2yr warranty, and P2.5k at Midas. It has provisions to attach a vent (required in a Miata), otherwise corrosive and potentially explosive gases can collect in the trunk. The trunk has these small rubber hoses that connect the battery to vent to the outside world.

The hold down clamp is 2 pieces. A horizontal piece that goes over the top, and a long vertical piece to hold down the horizontal piece.

One end of the horizontal piece goes into the sheet metal of the trunk, the other end is held down by the vertical piece. The sheet metal has 3 holes for the horizontal piece, the lowest hole is used for the original Panasonic battery. The top hole is almost right for the AC Delco battery. It ends up
being slightly too tight, in holding down the battery.

2 options: enlarge the hole in the sheet metal, or bend the end of the horizontal piece slightly (the end that goes in to the sheet metal). I preferred option 2 because option 1 would require rustproofing the enlarged hole with metal rust treatment and/or paint. Bending the top piece is easy. I used a hammer and the edge of a concrete step. You only need to bend by around 1mm
so go slow.

After that, next problem is, the vertical piece is too short. You need to fabricate an extension about 2 inches long. I made it by bending a very stiff, thick galvanized wire into a tight "S" shape.

Last problem is that the rubber vent hoses don't fit the battery vents, which is larger than the original. I had some plastic hose on hand that I cut into 2pcs of 2 inches each. It very neatly fits onto the battery vents, then the Miata rubber hoses fit into the other end of the plastic hoses.

Voila. No need to spend P13k (?!!!) on a factory battery. Above efforts took me 1.5 hours.

And special thanks to Joey P. for discovering that the battery fits.

-Jason