I saw the same question but it's always regarding professions like:
a) As teacher, I meet many children.
b) As a teacher, I meet many children.
In this case I would automatically say b) because it's what sounds better and is in fact the correct answer, for what I've gathered.
But my question is not profession related and I've seen it written in both ways:
a) We can use my body/floor/wall as canvas.
b) We can use my body/floor/wall as a canvas.
I feel like the first one sounds better but maybe I have it all wrong, perhaps because in my mother language we can say it both ways and we don't have to use "a" because the translation would be something more related to "one", and in this case i'm talking about the canvas more in an abstract way. I don't know if this makes any sense.