Rashi: Now he measured the width of the Bayis in the south, just like he measured in the north. It says "Achareha" ? this is like Acharonis (backwards), for he backtracked to the west.

Radak: Just like he measured the length of the Bayis and Gizrah, so he measured the length of the Binyan "Al Pnei ha'Gizrah" (in the east) and "Asher Al Achareha" (in the west).

Malbim: Now he measured from east to west "ha'Binyan Al Pnei ha'Gizrah", i.e. the cells, which are called Binyan. First he measured only 90, until the end of the cells. Now he measured "Asher Al Achareha va'Atikeha". There were Atikim (refer to 41:15:2:4) in the west. There were 29 Amos from the western cell until the wall ¹ of Chatzer ha'Penimi, for there were no cells in the northwest and southwest corners, like I explained (6). They stretched planks from the roof of the cell Achorei Beis ha'Kapores (in back of Kodesh ha'Kodoshim) to the wall of the Chatzer ha'Penimi, or from the western wall of Chatzer ha'Penimi to the roofs of the northern and southern cells, and built tall buildings on it called Atikim. They completed the Binyan above, in place of what was empty below. With these, the measure was 100 from east to west on the north and south sides of the Heichal, just like in the middle.

Rashi citing Targum Yonasan: Its corners. I.e. with Beis ha'Chalifos in the corners of the Bayis.

Radak: This includes cells and chambers. Each is called Atik.

Vilna Gaon: They are walls from the wall of the Binyan (Heichal) to the chambers, to strengthen the walls. Any wall made to strengthen another building is called Atik.

Malbim: It is a high building on pillars. Or, they put planks on the roofs of two buildings opposite each other, and build a tall building on top of them.

Radak: Aleph, Hei, Vov and Yud are interchangeable.

Radak: To the north and to the south ? the cells and chambers in them.

Vilna Gaon: There were Atikim on both sides (north and south).

Rashi: This does not refer to the Reisha, rather, to the coming verse. Ha'Heichal ha'Penimi is Kodesh ha'Kodoshim; Ulamei he'Chatzer are "v'Alemos Saviv Saviv" (40:30). For all these, "ha'Sipim veha'Chalonim ha'Atumos veha'Atikim Saviv li'Shlashtam" (16) ? all three, i.e. Gizrah (Heichal), Ulamos and Kodesh ha'Kodoshim, they had Sipim (Mezuzos), windows Atumos (refer to 40:16:1:1-5), and Atikim. I am unsure what these are; I think that they are square pillars that stick out of the wall for support.

Radak: This refers to verse 17. Even though the Navi gave measures for the length and width of the entire Bayis, ha'Heichal ha'Penimi (Kodesh ha'Kodoshim), Ulamei ha'Heichal ¹ and everything listed here had known Midos by themselves.

Vilna Gaon: Together with the Atikim, their measure was 100.