So youre going to make the enclosure like the one in the link?

For those that cant see it, its an enclosure using the spare tire well...but without fiberglass. Without saying what a ghetto idea that is... h: You can measure the space in the spare tire well with something like packing peanuts. You fill the well with peanuts, then take them and put them into a box. Then measure the dimension of the box. Its easier than taking a ruler to measure the irregular shape of a spare tire well.