Hiya!
After some more googling, I found this topic on the forums. By setting up a new project with their intellij plugin and selecting both sponge and forge, it set up a build file I was then able to use to build my plugin properly 
Minecraft Dev for IntelliJ - #29 by SparkVGX
Now I just need to figure out how to make it start a spongeforge server in the environment, but I’ll ask about that in their own topic.
Thanks all.