Questions - (kinda dumb, really)

I have a staggered wheel set up..

Front: 15x6 rim w/ P205/60/R15 tire
Rear: 15x8 rim w/ P295/50/R15 tire

If I wanted a "spare tire" (road trips) how would I go about it?
Is it even possible?
By default it would have to be front wheel size, right?
Lock differential, 1in drop in the front, will it have any affect?

Should I opt for a squared set up instead?

Or better yet, just forget about the damn thing and throw a plug kit and air compressor in the trunk?