Building a Mind Palace, at least the way it was written, was a long and arduous process. It took years for some, while for the talented few, it took several weeks.
Even then, Xuan was expected not to finish this for a long, long time. Except... Didn't he already get through the hard part?
With that in mind, Xuan imagined a temple appearing out of nowhere, as well as some footing to stand on. As expected, it appeared. There was furniture, some good food, and even a nice incense pot in the very middle that his imagination pretty much filled in.
Now, in terms of the Mind Palace itself, Xuan went ahead and imagined a large library, much like the ones he had passed while on his way to school. They seemed very practical, and that was the kind of basis he would use.
Or rather than basis, it was more like a foundation? Since he was a cultivator, it would take a lot longer for him to die, which also meant he'd have a lot more memories than normal people.
