I have taken a pre-made template by a guy in PSD and converted it to Affinity Designer format, with embeded Icon in 1024 x 1024 which you just double click for editing. I took the idea for embeded editing from another template here for IOS (green tech I think? Thanks!).
All credits listed in copyright of the file.
To edit, just double click the massive icon and boom.
latorante got a reaction from matt in OSX App Icon Template (easy editing, ready made slices)