Sentence Structure

Hi everyone,

I have some difficulty understanding the following extract from William Faulkner's As I Lay Dying:

When I reach the top he has quit sawing. Standing in a litter of chips, he is fitting two of the boards together. Between the shadow spaces they are yellow as gold, like soft gold, bearing on their flanks in smooth undulations the marks of the adze blade: a good carpenter, Cash is. He holds the two planks on the trestle, fitted along the edges in a quarter of the finished box. He kneels and squints along the edge of them, then he lowers them and takes up the adze. A good carpenter. Addie Bundren could not want a better one, better box to lie in. It will give her confidence and comfort. I go on to the house, followed by the
Chuck. Chuck. Chuck.
of the adze.

In particular, what is meant exactly when Faulkner says: "He holds the two planks on the trestle, fitted along the edges in a quarter of the finished box."? The two planks being fitted along the edges I can understand, but "in a quarter of the finished box"? I have tried imagining it, but for the life of me I just cannot conjure up any image to fit the words.

Any help from the teachers here would be most greatly appreciated!