Scudney :
I bought a set of afertmarket alloys shod with winter tyres when I bought my 2nd hand IS 6 years ago. By discarding the polystyrene packing, toolkit etc, there was room for the normal sized wheel/tyre which is what I now keep there for the spare. It also needed a bigger cover (floor to the central boot area) to the wider area, which I made up from black felt stapled to plywood cut to size with a jigsaw. Altogether, it looks much like the original boot floor so I was pleased.
CAUTION : My car is a 2008 base model, the tyres are 205.55.16, so thinner than yours. However there is still about 1 inch between the top of the normal wheel and the underside of the new base to the boot, so should accommodate a 225 width tyre. I cannot be sure of any other differences between the cars.
Good luck.
Geoffish