"Insert" a custom template into a specific forumview
 

I want to add some custom content to just a couple of forums and I'd like to have the custom content as a template so we can easily alter it.

I'm assuming I need to do something like this in the FORUMDISPLAY template?

If this_forum equals List of forums(1,2,3) then insert custom_template

Anyone help me out with what specific code I need to use?

You can create a copy of your regular template, edit in the extra content and set the desired forums to display that template and override user choice.