You may want to consider a battery tender. A master kill switch located on the batt is another option. If you just disconnect, do the neg thermal. Some products to consider
http://www.sears.com/shc/s/p_10153_12605_02871219000P
http://www.supertruckusa.com/products/99966_00.htm
The palce for Miata info: http://www.miata.net/