Rashi on Exodus 30:3/רש"י על שמות ל׳:ג׳

1את גגו THE ROOF THEREOF — This altar had a top but the altar of burnt-offering had no top, but they filled in the hollow space enclosed by its sides with earth wherever they encamped (cf. Mekhilta).

2זר זהב A CROWN OF GOLD — an emblem of the crown of priesthood (Yoma 72b).