Here I'm removing the amount I measured off the end.
 

Now that all the side pieces fit like I want, I'm milling a groove in them that the lid will fit into. I figure it would be better to have no hinges on this project for two reasons: first I think it would be easier to just remove the top and set it aside rather then deal with the top attached which might end up getting in the way. And second, little fingers won't get hurt if the lid were to come down accidentally.

Now the lid or top will have this configuration that you see below. But the bottom will have the opposite so they fit together. And I'll be making the lid fit nice and easy so I'll be adding some extra clearance to it. The reason is so Audrey can get the top on a little easier.
 

I know this is hard to see here but if you look close (arrow), you can see a radius that was left from my 1/2" diameter cutter while machining this groove for the lid. I'll need to reduce the size of that radius a small amount so it will fit my other half.
 

What you see now is a much smaller radius that should work much better now. This was planned because using that larger cutter removes material much faster than a smaller one. Then once the groove is to the dimension that I wanted, I changed cutters (1/8" diameter) and picked out the corner.
 

With the use of a file, I was able to remove a small amount on each corner so the two halves would fit together. Notice I removed the teeth on one portion of the file (with my belt sander) so I didn't leave any marks on the adjacent surface (arrow).