create file that can be mounted
To create a mountable disk image, use hdiutil. The straightforward way just creates a DMG which can then be mounted and written to
hdiutil create -size 500m -volname CLONEZIL -fs FAT32 clonezilla.dmg

To create a DMG with the content of an existing directory use
hdiutil create -srcfolder my-clonezilla-dir -volname CLONEZIL -fs FAT32 clonezilla.dmg

To mount it afterwards either double-click the DMG file in Finder or run one of the following in Terminal
open clonezilla.dmg
hdiutil attach clonezilla.dmg

To unmount either eject in Finder or run
hdiutil detach /Volumes/CLONEZIL