I want to make a custom icon for my dock app based on this excellent post (works on Big Sur).
However my custom image is a square. From where I can get a rounded mockup or is there an online generator to upload my image and pull the icon?
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Image2Icon free version has a great Big Sur icon style creator: https://img2icnsapp.com/. It works well and you can create big sur style icons in seconds from non-big-sur PNG icons.
As mentioned Apple Design Resources https://developer.apple.com/design/resources/#macos-apps contains templates for app icons
As well as templates for document icons.
They also have a seemingly complete cutout of all of the macOS UI elements which may be useful when designing interfaces on the whole
Check out the Image2Icon app, it solves the problem for me. Just drag and drop or copy and paste any file, folder, or app from Finder to the app's main window, choose the icon style, and then drag and drop that file again, the app will modify the original icon.