Rashi on Deuteronomy 10:1/רש"י על דברים י׳:א׳

1בעת ההוא AT THAT TIME — At the end of forty days He was reconciled with me and said to me, פסל לך HEW FOR THYSELF [TWO TABLETS] and afterwards, ועשית לך ארון עץ MAKE THEE AN ARK OF WOOD. I, however, (see v. 3) made the Ark first (Midrash Tanchuma, Eikev 1), because when I came with the tablets in my hand where could I place them? Now this was not the Ark which Bezaleel made for the Tabernacle, because with the Tabernacle they did not occupy themselves until after the Day of Atonement, for only when he came down from the mountain on that day did he give them the command regarding the construction of the Tabernacle, and Bezaleel made the Tabernacle first and afterwards the Ark and the other articles (Berakhot 55a). It follows, therefore, that this was another Ark; and it was this, that went forth with them to battle, whilst that which Bezaleel made went forth to battle only once, in the days of Eli — and they were punished for this, for it was captured by the Philistines (cf. Jerusalem Talmud Shekalim 6:1).