I’ve always used roto casting too, with spray ops for any deco. Like Ray mentions, you really need to watch out for some of those materials, especially including Phthalates, which are commonly used to soften material used in roto casting.
You’ll also have to consider mould if you’re considering anything that goes anywhere near water.
As an aside to the manufacturing of the outer, if you’re looking for it to make any noise you can have some fun playing with different bellow sizes and different reeds for a great range of sounds! And they don’t all need to be reeds, they can be compression and even things that you bend that make unusual sounds! Great fun to play around with, really makes you feel like a kid!