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Consider that the ark of the covenant, old Testament, was built by a specific design that God made in order to carry the "Word of God" tablets.
Likewise, the Living Flesh Word of God, Jesus, that was carried by the living Ark, Mary, was also built by specific design. Her being sinless was a specific design requirement by God Himself.

The scriptural EVIDENCE is contained in the Gospel of Luke Chapter 1 verse 28. The angel addresses Mary as "full of grace". Grace is the key word. A sinful person is not full of "grace". The term grace here refers to a person being full of God.
Note: the angel's greeting occurs prior to Mary conceiving Jesus.
As far as how she was created without sin, well, God is not limited by time. Jesus did in fact "save" Mary, and His salavtion victory on the cross was applied to Mary at the moment she was conceived. All this to acheive God's design of the perfect Ark to contain the Word of God.
Praise be to God

The angel addresses Mary as "full of grace".

Every redeemed person is "full of grace". And without the fullness of grace none of us will see Heaven.

Colossians refers to believers (not just Mary) as possessing God's "fullness" quite a few times. When a person is saved (as Mary realized she was in the Luke passage) he/she has come into God's fullness.

As far as your ark discussion, that is allegory and is a nice picture but I don't see anything close to such a comparison in the Scriture for Mary. I've heard a few Catholics say such but I think they're relying on something that's not even implied anywhere in Scripture.