Anim Trail Dissolve Material

I am trying to have a trail dissolve away as it extends, with some intermittent holes appearing in the middle ( so it kind of looks like a ragged cape), but the only way I found to do it is really inconsistent (using the reflection vector as an alpha doesn’t give controlable results). As usual, I’m probably doing things much more complicated than necessary. If anyone can give me a hint of how to do this, i’d appreciate it.
Here is what I want it to look like:

Here is what I am doing now and what it looks like: