It is very fussy. As its freezing the ambient moisture in the air rapidly, it's not always certain it'll achieve the shape you want. Hence why it's easier to simply let it spike...

[As he spoke, Byleth held his palms close together, manifesting some ice magic. He was experimenting with different shapes he could make in miniature, to get a better feel for what might be the most refractive upscaled. Eventually, he settled on a somewhat hollow orb with roughly cut edges.]

I'll make something like this. Do you think you could have some light magic glow inside of it once I create a bigger version?