These are parts that i designed to adorn a wooden Xylophone that i but for my daughter. The mallet heads should be printed at about 30% infill or better to give them some weight. (i did 50%)
the handle pieces need small wood screws to attach to the instrument and will hold the mallets tightly at first but break in quickly. i would recommend 50% here again due to the strain and stress of a small child playing with it.
As for the caps, they are designed to go over the stubs that stick up through the keys. my daughter never let me add them cause she likes to take the keys off and play with them like dominoes....
Cheers and happy printing!