Here I'm applying glue to the miter joints and bottom groove. The trick here is to apply only the minimal amount of glue to the miter joint. This glue is just there to hold the box together until the miter keys are installed. Excess glue will make it really difficult to pull the joints together tightly.

To allow expansion/contraction, the top is left floating, except for one tiny drop of glue in the center of the box ends. This drop of glue locks the top at its center position, yet still allows expansion and contraction on either side.