lots of ways to do it. All of them varying in complexity and accuracy, depending on what you looking for.
You didn't give a lot of info. so this is what i would do (with so little to go on). Get a smaller peice of material (or cut that one down with a band saw). Then since you said it was to be a box, Mill the inside of the box to the right dimension in a vice, then take the part out flip it over clamp on the inside of the box by expanding the vice. and cut the outside to shape.
This is a simplistic description, and its only one way to do it.
Another way is to mill the whole thing from the top but, on the outside of the box mill down to .875" (leaving an 1/8" for the vice to hold on to) and then flip the part over and mill off the excess.
And if your indexing skills are shaky or non-existant then make an 1/8 in round over on the bottom side to blend the two operations.
Well, My job is done here! These explanations are probably clear as thick mud! Enjoy!