Hello all... Jim here - designer of the makeamarimba instruments.
Here's an idea I had for the P1 shortly after I finished the building guide, which is why it never made it into it by the way. These little cable clips are 6mm - the exact size of the mallet shafts so they are perfect for keeping mallet secure with instrument. The clips come in a range of sizes so even if you didn't use 6mm shafts you should manage to find a clip to suit. Notice that the rubber tips sit past the end under the end piece so that they don't distance the shaft from the timber - in fact because of this you can wedge them in to make an even more secure hold. You will also notice that I clipped the mallets at a slight angle so gravity would tend to hold the mallets in rather than pull them out.