Rashi on Exodus 25:17/רש"י על שמות כ״ה:י״ז

1כפרת — A cover for the ark which was made open on top. He laid it upon it, flat like an ordinary slab.

2אמתים וחצי ארכה TWO CUBITS AND A HALF SHALL BE THE LENGTH THEREOF, as was the length of the ark, and its (the cover’s) breadth was as the breadth of the ark (i. e. of the inner wooden ark, cf. Rashi on v. 11); thus it rested on the thickness of its four walls. Although it does not assign any size for its (the cover’s) thickness our Rabbis have explained (applying to it a ג"ש) that the thickness thereof was one handbreadth (Sukkah 5a).