How can I compose a poem related to Of Mice and Men?

One approach that might work in writing a poem based off of the novel is to compose it from a particular character's point of view.  For example, a poem from George, talking about what he did and how he views it, could be quite powerful.  I think that being able to express in a poetic form George's emotional state could lend itself to being compelling work.  The poem could enter George's mind and probe into the implications of what he did and having to live with the consequences of such actions.  The poetry of lonliness can be seen in this regard.

In a similar manner, entering into Candy's mind could also reveal a very sad poem.  Candy, finally broken in recognizing that his dream is destroyed with Curley's wife's lifeless body, could prove to be powerful.  Candy who is alone without his dog and devoid of his dreams could be a subject area where personal feelings of regret, anguish, and a sense of loss could emerge.  This might be where another poem that enters into the frame of reference of a particular individual in Steinbeck's work could be worthwhile.  Being able to write a poem based off of Steinbeck's work might function best if it is focused on a particular character and how they view being in the world as a result of their characterizations offered.

Can you name a published poem that connects to an aspect of Of Mice and Men?

A published poem that relates directly to Of Mice and Men is the poem To a Mouse, by Robert Burns. Published in 1785, the poem was, in fact, the inspiration for the title of Steinbeck's novel. 

But Mousie, thou art no thy-lane, 
In proving foresight may be vain: 
The best laid schemes o’ Mice an’ Men 
          Gang aft agley, 
An’ lea’e us nought but grief an’ pain, 
          For promis’d joy! 

In the poem, a farmer is apologizing to a mouse for inadvertently destroying its nest as he harvests his field. He tells the mouse that he meant it no harm, and that he believes himself and the mouse to be mortal companions. He is sorry that man's dominance over the earth and its creatures has led them to being in their current situation. 

Some questions that could be asked about the novel and the poem could relate to common themes, the significance of the title of the book (and why Steinbeck chose that particular line from that particular poem), or the characters themselves. Given the presence of mice and other small, helpless creatures in the novel, it seems likely that Steinbeck took more than just the title from the stanza in the poem.