The speaker imagines himself as a carpenter, armed with a simple box of tools, and announces that he will build many boxes. The tools are ordinary, but what he plans to make with them is not. One box is for serpents, another for devils, so he sets out to contain the things people tend to fear or want to shut away.
The last line changes direction. A box "like a suit I might dance in" turns the idea of a container into something worn and lived in, even joyful. The poem suggests that boxes can be for protection or play as well as for confinement.
The mood is calm, confident and a little playful. The speaker doesn't sound afraid of serpents or devils, only practical about them. Judging by the final image, he seems to see making things, whether to lock up darkness or to move freely in, as a way of taking control and finding some freedom.