I went with the Massive MODULAR Spaceship option, with emphasis on the Modular part. References dating back to the Tom Baker era to "jettisoning" of parts of the TARDIS, and even more recent references to the TARDIS rearranging itself (the moving swimming pool) seem to support some sort of modular design. And another key word is "massive", not "infinitely large" which is a bit of a misnomer. Again going back to the Baker/Davison era there were references to jettisoning a certain percentage of the TARDIS' rooms, and you can't have a percentage if it's infinite.
I suddenly came over nostalgic for, of all things, Second Life. During my brief (and ultimately disappointing) involvement with the virtual community I came across a number of folks who had created their own TARDISes, in some cases using designs not too dissimilar to some of the speculation here (maybe not hypercubes, grant you).