So you have to cheat : just put some duct-tape (scotch-tape, whatever...) on the HD Hole.
Tada, a proper DD disk with 720 KB (if your OS allows it).
Or a common 2x178KB with AMSDOS (stock CPC firmware) - if you have a manual side switcher button too of course to read second side of disk.
PC users used to do the opposite : file a HD hole on DD disk...this worked well sometimes.