Usually the walnut goes inside of the pincushion.
For cuteness and impracticality the walnut shell is going to be used at part of the pincushion.
Crack open that shell and eat the walnut.
Or get some one else to crack and eat the nut for you.
I used wool felt and stabbed that ball until it was firm and walnut shapeish.
I then glued the wool felt ball to the inside of the walnut shell.
They look like little snails.
And they seem to hold the pins a lot better than I thought they would.
Now to keep them away from the cats. They seem to be a cat magnet.