Spiral out the Alphabet!
Charcoal, 31 bytes

Ｆ⮌…⁰ＮＢ⁺²⊘×ι⁺³ι⭆α§α⁺λ÷×ι⊕×ι⁺⁹⊗ι⁶


Try it online! Link is to verbose version of code. Note: The deverbosifier outputs a trailing separator for some reason. Explanation:

Ｆ⮌…⁰ＮＢ


Draw boxes in reverse order of size (largest to smallest).

⁺²⊘×ι⁺³ι


Calculate the size of the box.

⭆α§α⁺λ


Draw the border of the box using a rotated alphabet.

÷×ι⊕×ι⁺⁹⊗ι⁶


Calculate the letter that would appear at the top left of the box (0-indexed).