You need a vented battery as you need to get any gasses out the boot.
The best and the correct size with vent tubes is the original equipment Panasonic from your Mazda Dealer or from Autolink UK circa £150. It must be connected to the vent tubes that should be in the battery box to get the gas out the boot.
If they are not there Autolink will have them but the local pet shop will have similar for fish tanks.
Look on their site to see what you need. The one in my car lasted 13 years.
There is a Lion 114 which is the correct size from Austraila and designed for the MX5 circa about £100 from Europarts there are no vents but the manufacturer advise it is designed not to need one. Good reports so far.
http://www.eurocarparts.com/ecp/p/acces ... 0&cc5_1026There is the Westco that gets a bad name from reliability, I had that problem and MX5 parts tested it for a week before repacing and I would not by one. MX5parts sell them.
The batteres from Halfords and most motor factors etc like the battery you have all do not fit properly to some extent or other.
Both the Panasonic and the Lion need a Smart charger. Keep an eye out for Lidl and Aldi as they sell the for £12 a few times a year.
If you want a battery that does not fit properly and can be used with your existing charger then just buy a cheap battery that does not fit from the nearest Halfords or car parts shop.
Your choice, the existing battery may possibly not fail the MOT, while the clamp will not fit, the MOT tester is allowed to give you an advisory if it is not likely to fall out it's battery box and that is the case with the Mk2