All thread / 2x4 / dowels on the inside
45s in the corners - my favorite for a box with a loose panel is to take the rattling side, create as big of a gap possible between it and surrounding panels, the pour some fiberglass resin in there. Once that's cured, do a couple layers of glass over that. (This works great for that by the way.)
And I don't like to use screws if I can help it. Too many in an effort to hold something together just chews up the wood and makes it a tad weaker IMHO.
Edit: All thread work great for spreading said panels enough for the fiberglass resin to soak into the crack. Then you can just use it as bracing.