You can turn your Boombox into a backAccessory, and then just attach it like any other backAccessory.
How does it work?
The tool “BoomBox” and backAccessory “BoomBox” is not gonna be the same, it is only gonna look the same. Think of it like a key laying on the floor in Piggy/Granny. When you pick it up, it’s not actually the key on the floor you get, but instead just a tool that looks like it. When you then loose that key, a model of the key will be placed on the ground again.
Find a backAccessory from the catalog, import it into your game, have a look at how the properties are, and replicate them. You will quickly figure out how come the BoomBox is how it is.