Your angle will be between 0 and 90 degrees.
The way I build my boxes, on this one I would:
3-Nail back and bottom together
4-Cut front bottom
5-Nail front bottom on.
6-Cut sides and nail on(For this, cut a peice 18" tall, 8" wide, hold it next to it, get it flush on all sides, mark the angle, then cut it. Make 2 of these pieces.
7-For the last peice, cut it the width of the box, by whatever approximate height to cover from top to bottom, plus an inch. Then hold it over the hole, flush, mark angles.
Then lay it on table and adjust to the angled line by eye.
I've always done it this way and with a little patience it comes out perfect.