Andrew has spelled this out pretty well, but if you do go ahead with this, keep in mind that the subwoofer that fits may well be a size smaller than the original woofer to work well with the box. i.e. if the woofer was a 12", a 10" subwoofer may be all that works in the box. Keep that in mind while running T/S parameters.
Many of us are in apartments and all our friends are in apartments, so getting to do any real cabinet construction is a rarity. Thank goodness we have a garage and a patio!
