Or HaChaim on Exodus 25:27/אור החיים על שמות כ״ה:כ״ז

1 לבתים לבדים, as housings for the staves. Whereas I have explained in connection with the rings of the ark that Betzalel had to cast four rings and attach them firmly to the ark, whereas he would insert a second set of rings inside the first ones to support the staves, the Torah here alters its instructions. Here there was no need to make a second set of rings. The rings mentioned here were the ones into which the staves were inserted directly.