>>8812962Mask of Adaptation gives you some physical changes, but they have to be in tune with the environment you're in. It doesn't seem you get to decide the changes, the mask picks what's best for you.
Sounds like a "Mask of Shapeshifting" is more in line with what you're going for. My first thought "Isn't that the Mahiki?", but I checked BS01 and I found out they're not even sure themselves if the Great Mahiki allows for actual body shapeshifting, or if it just creates realistic illusions.
If it were to be confirmed that the Mahiki only creates illusions, you could invent your own "Mask of Shapeshifting", which would grant the user the ability to actually change their physical body.