I may have forgotten one important step.
First use a program like 7zip to extract the .crx file.
After that you need to edit the manifest.json from the extension with your favorite text editor and make sure it has " "manifest_version" : 2" somewhere in it.
Save the .json then try adding it to Chrome and then it should work. It's been a while since I've had to do it so again, I forgot about this part.
The .json already is at version 2
Although kinda makes me think maybe the developer is hiding something, hence it isn't in the store but such a shame as the extension just worked simply and brilliantly.
It seems Chrome accepts the extension but when loading YouTube, no box is visible and as such, the extension isn't actually loaded.
It seems to get it to load properly, I have to refresh the YouTube page once it's loaded for the first time. Not ideal
Will look at other solutions.