I have not created such a file for a few years so you may have to do a bit of trial and error .....
create a file.
use the dd command to fill it with zeros to maybe 256MBs
The file is then 256MB in size
Then create an ext filesystem in that file using the command line mkfs
It should then be usable as a changes file.
But of course I may be missing some important part I do not presently recall EDIT
It seems I remembered correctly as I have just made such a file here,
I used the right click option in Dolphin to create the file.
I then used the dd command to fill it to the required size ...dd if=/dev/zero of=/path-to-filename bs=1MB count=100
which gave a 100MB file.
Then still in Konsolemkfs.ext2 <path-to-filename>
and elected to continue - ignoring the warning it gave.
After that I was able to loop mount the file and treat it like any other partition in the system.mount -o loop <path-to-file> <existing mount point>
I am unsure if other parameters should be used when making the filesystem ..... but I would try the above first to see if it worked ...... my memory is not very comprehensive regarding this as I only rarely used it, and then only for tests.