We've probably touched on this elsewhere but the box is unlikely to include perfect joins all round and so what I do here is switch to silicone to seal it up altogether. At the same time you can pop a bead of sealant down the outsides of the box where it about the cap-ends, principally for extra support (because they are anyway sealed from within).
Silicone preferred at this stage because (a) its cheaper (b) its lighter (c) its quicker and easier to apply. Nonetheless it produces powerful fumes in enclosed spaces and should at any stage you feel that you are being watched over by a giant rabbit then it is probably time to take a break.
