Don't know anything about fat jars, but you could try this jlink plugin:
https://tentackle.org/static-content/sitedocs/tentackle/latest/tentackle-jlink-maven-plugin/jlink-mojo.htmlIt works with non-modular projects as well - everything just gets dumped into a folder and put onto the module path. Just follow the standard Maven jlink guide:
https://maven.apache.org/plugins/maven-jlink-plugin/usage.html but use the above plugin instead. I've been using it and it works great. Hope this helps.