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I started out with a medium acrylic brown, mixed with a lot of water.
A. LOT. of water.
(I just got a small set of acrylic paints, nothing fancy)
Then I painted over the whole thing lightly with an even more watered-down darker brown, painting with the grain of the wood to make the texture pop.
After that, I drew on an eye, snarling mouth and tongue in pencil, which I then painted over with more acrylic paint.
Then I cut a piece of felt so that a few pieces fit the bottom, sides and top of the inside of the box. I used a rotary, but scissors work just fine.
And now all that's left is to have a player put in their d20 or 2, have them shake it up, and take a look to see if they get away with their shenanigans.