This box is sized to fit the rattles. I start with the pieces that fit under the lid. After assembly I added a shaped floor to make the box more of a custom fit. I should have cut the channels into the floor at this step.

It’s time to check the fit. The largest rattle had just enough room.

Now I can add the lid. The important part at this point is to get the width right. The final fitting can be done after the sides are added.

 

Now the sides can be added. Then the sides are sanded to match the front half (after glue up).

The top is trimmed and sanded to fit. Then it is taped in place for final shaping. No pictures of that - sorry.

A brass rod is cut to create the hinge pins and the back of the lid is shaped to allow it to open.

Then the sides are shaped into a continuous curve. The front gets some curvature and the corners are generously rounded.

Here is a copy of the drawing I used. I turned the large diameters at least 1/4" larger than the holes (1/8" per side). If I make more of these I might try for a thinner wall.  I also might make the rattle cavities larger.