OK, I did a few quick calcs.
Assuming you can fit a 185/60/14 wheel/tyre in your spare wheel well then you have a spare wheel well of approximately 23" diameter:
Tyre depth (mm) = 185mm*60% = 111mm
Total wheel diameter (mm) = 111 + (14*25.4) + 111
Total wheel diameter (mm) = 111 + 355 + 111
Total wheel diameter (mm) = 577mm
Total wheel diameter (in) = 22.7" (say 23")
1 cubic foot = 1728 cubic inches (12*12*12)
Assuming you were running a JL8W3v2 with a required enclosure volume of 0.79 cu ft the enclosure volume, expressed as inches, would be 1728*0.79 = 1365 cu in
A slightly rectangular enclosure (totally square boxes are not as good as those which are not, as the sound can cancel intself out) of EXTERNAL dimensions of approximately 17" * 14" will fit snuggly in the spare wheel well (IE a rectangle within the circular spare wheel well).
Assuming you are constructing in 0.75in material, these dimensions are reduced to 15.5" * 12.5", giving an internal surface area of approx 193 square inches.
If we divide the required volume of 1365 by the surface area of 193 we arrive at the required INTERNAL height of the enclosure, which is 7" (will measure 8.5" externally). This figure should be increased slightly to make allowance for the volume displacement of the driver itself.
Now, obviously, if you aren't using a driver with such a low enclosure volume requirement you will need to increase the height of the box - all you need to do to achieve the look you want is to increase the height of your new false floor accordingly, so the enclosure top and the new boot floor are to the same level. The boot floor is then cut to go around the (rectangular) enclosure and the whole thing can be carpeted as one item - this saves all the hassle of trying to build a round box, or cutting a round false floor around it etc etc. It also means you do not have any sealing issues etc, as its a simple near-square enclosuredropped into the spare wheel well - which also means it will take 10 Min's to tally remove from the car and return it to standard.