I have an interesting idea. And I'll use this model (that I have to remake cause of its height) 
But you can make a little option where you can change the projectile spawn position (but using the center of the voxel itself) in this area
You can choose the color of the projectile that will spawn and whether or not to make the voxels invisible or visible.
I chose Orange for this example. But the arrow would come from that point like so;
