Rashi on Numbers
המשכן THE DWELLING — i.e. the lower curtains (those which were spread immediately over the boards of the Tabernacle).
Siftei Chakhamim
The lower drapes. Why were they called the Mishkon — because they were seen from inside when one stood in the Mishkon. Furthermore, the Torah writes “You shall make the Mishkon — ten drapes…” (Shemos 26:1) which refers to the lowest of the four covers, not to the Mishkon as a whole, as in “You shall erect the Mishkon” (Shemos 26:30) (Re’m).
Chizkuni
המשכן, this refers to the carpeting of twisted linen which formed part of the roof of the Tabernacle.