Hi there! So here's the issue with the Tardis itself being the subarchway: If it was the subarchway, then you would have access to the inside of TARDIS but not be an outside functional object because the subarchway would have to be mobile to do that. The subarchway would need to be a time/location that the Doctor could retrieve the TARDIS from, with the TARDIS as an object within it. I'd recommend taking a time/place from canon that he knows the TARDIS will be present at.