my only issue with doing a spare tire box is the sub firing direction.... firing any sub upwards will give you maximum trunk rattling with less bass.LOL
if ya dont wanna take up much space then id say buy smaller sub or subs that use small box space...
if ya do 2 10s heres an idea...
pull your trunk carpet out on the sides and build a box to go as far back as posible and build the box kinda like a triangle shape so the subs would fire back inwards and towards the front.. kinda like my side panels i built... BUT they would be the whole box and just angle it back as far as you can at that angle....

they are only slightly angled....
something like that....
i think thiers quite a bit of room for a nice box for a 10s one on each side.
that would use the least ammount of space...
you could even notch the one side braces and be able to make the box alittle larger... JUST be sure to brace the metal around the uncut pieces OR when you build the box you could also just use some angle iron around the side of the box and screw the angle iron to the side panels... which would brace the steel as good as stock and also brace the box so it wont move as well

you would also want to brace the box to the floor as well.... then just carpet the boxes to somewhat match and then you got yourself 2 nice boxs....... tucked away leaving you lots of space in the trunk and also still alowing you to keep your spare tire where its at
what ya think?
i just came up with the idea.LOL
tho my sub need to much room to do that.LOL
tho a 10inch sub on each side would be realy nice! depending on what model subs you get of course....