Sorry, but this is one of those plainly answered in the rulebook things. The Equipment section has musical instruments on the gear table, they are listed as a two handed item. There is no alternate one handed version listed.
Also my Dad has been a drummer for many years, I have seen him play many times and multiple types of drums, and let me just say FORGET about playing a drum effectively with one hand.
I mean I'm sure there's some one-handed expert drummer out there going viral on YouTube or something, but that's a heavy exception.
But yeah, it's made fairly clear that unless your GM permits otherwise, you are using both hands should you choose to bring out an instrument.
The idea of attaching drums to a belt is cool flavor, but since doing it just to free up hands is kinda just min-maxing I would rule that you have to have your hands free and use an interact action to shift it into a playable position, and then while it's there it's too inconveniently placed to effectively use your hands elsewhere. Basically make it mechanically the same as drawing, holding, and stowing another instrument but allow the flavor.
