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 :).Image 2
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.Image 3
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).Image 4
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...