I don’tthink you d oneed a vented battery,
a) the Mk2 MX 5 has an extractor vent in the boot on the left hand side, that vent will remove any possibility of fumes from charging a closed cell battery as the vehicle drives along. If you are charging the vehicle when stationary, the it would always be better to leave the boot lid open. MX-5 parts offer an alternative to the Panasonic battery which is gas-free (although the reviews suggest it is not worth the money):
Battery, Westco, MX5 Mk1 2 2.5
b) at some stage in the past Mazda service switched to supplying an 063 type battery as the replacement, apparently due to problems with sourcing the original compact Panasonic battery.
I have a standard Yuasa 063 glass mat type 45Ah battery and this replaced a similar Bosch Battery (as recommended by Halfords for fitting to an MX5 Mk 2.5). The Panasonic worked 2003-2011 ( 8 years) , the Bosch 2011-2021 ( 10 years), trickle charging on both when not regularly used . Because these batteries were a bit larger than the Panasonic I had to saw off the L-shaped clamp bar off the clamp hook and use a bit of rubber mat to retain the battery. I also fitted a piece of rubber mat instead of the battery tray, which is only there to facilitate factory installation. I did not attempt to fit the vent pipe and have seen no signs of corrosion in the boot in 10+ years. As the 063 batteries are half the price of the Panasonic, and have higher cranking current, it was worth the effort. Just make sure that the terminals are the same way round Pana. There is a thread on this topic:
Battery change MK1.