A little over a month ago, I've released a Blender addon, called DECALmachine.
Check out the trailers:
Have a look at the documentation, which covers everything you need to know, including installation, how to use it and Custom Decal Creation.
If you encounter errors or problems, please see the FAQ.
If you experience a problem not covered, please report it.
For suggestions, feel free to comment below, eMail, or post on Blender Artists.