For tools this one's simple. You'll need:
Now, first just cut off a chunk of cardboard a bit longer than your shelf needs to be. I ended up needing two layers to keep it strong enough. You could even get fancy at the end and tape 'em together for a more cohesive shelf. You probably noticed mine isn't exaclt perfect, but it does the job :).
Next, lay it down in front of your milk crate and mark about a 1/2 inch out from either edge of the box, to give your fingers
some support room. Mark it on one or both pieces and get 'em onto your cutting surface.
your marks (from the manufactured side, not the side you cut off the box, as that's probably not perfectly straight unless you were more careful than I was).
Cut 'em off! You can use the marks you made, or use the first piece as a guide for the second so they end up the same length. Next: joinery...