How thick was the lid of the Aron?
Which the Gemara in Sukah, 5a learns from the smallest of the Keilim - the Misgeres (the frame around the Shulchan), which was a Tefach wide, as the Torah specifically states. See Torah Temimah, note 21 (or via a Gezeirah SHavah "P'nei" "P'nei" from "me'eis P'nei Yitzchak Aviv" in Bereishis 27:30 ? See R. Bachye and R. 'Chavel's footnote footnote DH mi'Gezeirha Shavah ? ').
Refer also to 25:11:2:1*.
Why was the Kapores made specifically of gold?
Yerushalmi Shekalim, 1:1: Because 'Let the gold of the Kapores come and atone for the gold of the Golden Calf'. 1
In fact, the D'rashah actually incorporates all the other Keilim in the Mishkan that were made of gold. See Torah Temimah, note 22.
What did the Kapores correson to?
R. Bachye (citing Bamidbar Rabah): The Kapores on the Aron corresponded to the Serafim 1 - consultants to the Shechinah - on the Kisei ha'Kavod. 2
R. Bachye: And the two Keruvim corresponded to Heafven and earth, where Hashem dwells.
R. Bachye: As the Navi writes in Yeshayah 6:2 "Serafim Omdim mi'Ma'al Lo".
