SeaGate Expansion Hard Drives are thin enough. The Free Agent is thinner and can has more storage but it’s more expensive. Click here
As for its compatibility with MacOS, @johnpowell (the composer?) already answered that. If you want it to work with Windows and MacOS, choose the FAT option instead on Disk Utility. (FAT does not make it only work with 16-bit, it’s actually more flexible, it can handle 16, 32 and 64-bit as compared to Windows FAT32 format).
Hope this helps :D