That is the tutorial you should use. Yes, it requires you to modify the original template files. No, your cannot use /custom folder to make these changes.
This is one of the main reasons that we introduced Gantry 5. In Gantry 5 you can add as many module positions as you like, wherever you like, without having to modify files (it's simply drag and drop). There I no direct migration path from a Gantry 4 template to one using Gantry 5 though... you have to redevelop your site.
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