The first stanza is 8 lines long each line alternating between 4 and 3 beats per line. The first stanza pictures the box in the readers mind, describing in great detail and how it was made. The stanza is describing something that has happened maybe many years ago, but it is still cherished by the writer. If a tune was added to it, it would make a good song as the words flow off of your tongue. The rhythm is in lines, the first line is 4 beats then 3, 4, 3, 4, 3, 4, 3 beats.
There is no set rhyming scheme, words rhyme in different places; most of the rhyming words are half rhymes rather than fully rhyming words. The box has many different meanings not just the one that is displayed on the surface. It is a gift from her lover to her, it represents their love for each other. She says that a lot of time was spent on making the box just like a lot of time was most likely spent trying to start and improve their relationship. The poem is comparing nature to their box and relationship and the time taken.
The second stanza is 8 lines long each line either 4 or 3 beats long same as the first stanza but they are not in a pattern. This stanza explains their relationship and the twelve black books she wrote it in. She is using metaphor's to explain their relationship such as 'sanded, oiled and planed' which is how they wore away each others fears about each other, 'planted a garden, built a wall' which is how their relationship started and went on, 'seen jays and goldcrests, rare red kites' how they told each other their rarest secrets that they won't tell anyone else.
The speaker is speaking from an experienced point of view. The stanza is saying that if it doesn't come quickly it's not worth having. The third stanza is the same length as the other two stanza's and has the same style of beats, in the same pattern as stanza one. The stanza is about the box and how she keeps it unlock in full view of everyone to come and see it and read it. She must be close to death as it seems she is referring to everything she has done to make her think about her life so that she is pleased with her life.
The last two lines re-write the things they did together. Its most likely that her lover has died whilst she was making this poem and she wants to die to be reunited with him. He was still alive for the first two stanzas but died before she could write the third. Maybe it was a poem as a get well poem to him but he suddenly became very ill, as the third poem doesn't seem to fit into the style of the first and second stanza. It is a very moving poem which is very effective.