Actually the way I am proposing is how Ubuntu's USB media creator used to make USB keys. It's new to 18.04 that it does them the way it works now with two partitions.
Two important things to keep in mind when making a FAT32 partitioned USB key:
1. You need to "extract" the ISO image. Don't copy it. You should see a directory "casper/" and another directory "efi/" in the root of your USB stick.
2. Make sure that you create a partition like I said with Gparted. Windows likes to sometimes format the USB key without a partition table, which can lead to problems.
Just copy the deb file into that directory I indicated.