How can I cut approximately 1/4/-1/2 inch off of a 3x3x3 block off wood?

I'm making a motor mount for a dolly, and To do this I cut the shape of the motor out some blocks to form a 3"x3"x3" cube with a slot for the motor in it. The place that the mount has to fit into is just a bit smaller than the mount itself. How can I cut a small amount off of one edge of the cube?
Also, I only have some basic tools (Hand saws, skill saw, Miter saw)

Burf6 years ago
Carefully measure the amount you want to remove from the block and mark across with a square. Using the square transfer the mark to the opposite face of the block.
With the miter saw, cut to the full depth of the saw blade (I always leave the pencil mark after the cut and dress it down with a sanding block when I need a precise cut.) Turn the opposite face up and again cut to the pencil mark you made there. Then sand it down to the precise dimension.