Take your time, there's no rush. Your project will still be there next week, next month, next year.
Another comforting thought is: if you don't want to Do It Yourself, or if you can't find a satisfying and non-frustrating way to Do It Yourself, other options do exist. One option is: don't do it at all. Another option is: find someone else to do it for you, and compensate them as required.
Previous posters have suggested ways not to do it (cut a rectangular or a six-sided hole in the rear panel) at all. Another way, not yet mentioned, is to drill a simple circular hole in the panel and pass the AC mains cord through the hole. Add strain relief, add yank-out protection, add friction protection, done.
Another comforting thought is: if you don't want to Do It Yourself, or if you can't find a satisfying and non-frustrating way to Do It Yourself, other options do exist. One option is: don't do it at all. Another option is: find someone else to do it for you, and compensate them as required.
Previous posters have suggested ways not to do it (cut a rectangular or a six-sided hole in the rear panel) at all. Another way, not yet mentioned, is to drill a simple circular hole in the panel and pass the AC mains cord through the hole. Add strain relief, add yank-out protection, add friction protection, done.