jtonline, a moderator here, has an article on his website about fitting a 18" full size alloy spare wheel to a Sport Nav with Bose - check out http://www.jtonline.info/18-inch-spare-wheel/
There is also a cheaper option of fitting a 16" full size steel wheel and suitable sized tyre which will have virtually the same rolling radius - check out this website for examples :-
https://www.tyres-outlet.co.uk/cw/bundl ... Type=steel
This costs around £125 for the steel wheel and Toyo tyre combination. I wish now I'd investigated and gone down this route rather than the Mazda dealers space saver option. NB: You will still need the various jack and some other items mentioned in the article above.
NB: Remember if you have a space saver wheel and you get a puncture on a front wheel then you need to move a full size back wheel to the front and fit the space saver to the back so an additional hassle.
