How to? Decals only visible in a custom volume

Hey there,
a few days ago I had a question how to create custom volumes, you can find there:…port-as-volume
Thanks @FrankieV for the answer^^

Now I have played around with decals and I have no idea how to do it or if it´s possible…
I need decals only visible in my custom volumes to show the attack range of towers who can get built anywhere, but if there´s a higher area in its way, the complete higher area gets highlighted, instead of note where the tower can shoot.

So, the question is, how can I make the decals only visible in the overlap volume I have created before?