Teaser 2897: Easter egg

From The Sunday Times, 1st April 2018 [link]

“Granddad, I’ve got a great big Easter egg in a square box!” My grandson Liam inspired this Teaser in which I have used non-zero digits then consistently replaced them with letters, using different letters for different digits.

In this way, I have:

EASTER
(EGG)²
BOX

Two of the numbers add to make the third, and if I told you which number was largest you should be able to solve this Teaser.

Find the value of BOX.

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