Creating ISO image with directories using UltraISO command line

I’m trying to create an ISO image containing several directories and files. The directory structure I’m trying to create in the ISO image is NOT the same as what is currently on my hard drive. This process will be done by another script, so it must be done with the command line.

Example hard drive format:

a.txt
b.txt

What I want the ISO image to look like:

aFolder-|
        |-a.txt

bFolder-|
        |-b.txt

According to the help page for the UltraISO command line arguments, I can pass -newdir dirname to create a new directory in the ISO image. I should also be able to use -chdir dirname to change directories, but it’s unclear how this works and I can’t seem to get it working right.

At the moment, I am just trying to make aFolder and put a.txt inside it. These are the commands I’ve tried with no luck.

ultraiso -imax -l -newdir aFolder -chdir aFolder -f a.txt -output test1.iso

This creates aFolder but places a.txt at the top-level (next to aFolder but not inside it).
ultraiso -imax -l -newdir aFolder -output test2.iso followed by ultraiso -in test2.iso -chdir aFolder -f a.txt

I thought that maybe the folder had to be created with one call, and then changed into the next, but this gives the same result as 1: a.txt is next to aFolder.
ultraiso -imax -l -newdir aFolder -output test3.iso followed by ultraiso -in test3.iso -chdir aFolder and then ultraiso -in test3.iso -f a.txt

Then I thought maybe there’s something internal to the ISO that stores the current directory, and that a command has to set it before you can put files inside… Nope, same as 1 and 2.
ultraiso -imax -l -newdir aFolder -f a.txt aFolder -output test4.iso

Maybe I could do -f filename_on_disk filename_in_iso? Nope, this command doesn’t even work: wrong parameter: aFolder.

So how can I make a directory in my ISO image and then place a file inside that directory? I would prefer to continue using UltraISO but I am open to other suggestions.