I was looking for phone cases and thought a cassette tape/player would be cool, since a tape and a phone are similar in size. If you want a retro look for your phone, maybe give this case a try.
I'm also working on a walkman-inspired cassette player case, since it would make more sense as a phone case. Having your headphones plugged in to a Cassette player looks more normal than plugged in to a tape IMO.
Just to be safe I would recommend printing this with a slightly-more flexible filament to prevent the corners or sides from snapping off when putting your phone in.
UPDATE: Uploaded two template photos for the label, cut out the gray parts and add to the case!
Found a free-to-use online base model for an iPhone 6 case, and worked off of that in Tinkercad. I tried my best to be accurate to the look of a Cassette tape without adding too many bits that stick out from the case since I prefer my phone cases to be smooth on the sides.