Help making .ISO bootable bios and UEFI

[larry]

I'm trying to make a iso, based on Ubuntu 18.04, that boots in both Bios 
and UEFI mode. The application embedded in the Ubuntu base is not 
installable, it is designed to run from a USB flash drive. I have no 
idea how to use grub-mkrescue in this case. I've was trying to use 
grub-mkimage and xorriso to accomplish this without success and it was 
recommended that I try grub-mkrescue. The grub.cfg that I built follows:

#grub.cfg
set timeout=3
set color_highlight=black/light-magenta

menuentry 'Foxclone' {
        set isofile="/efi/boot/foxclone025-02.iso"
        loopback loop $isofile
        linux (loop)/casper/vmlinuz.efi boot=casper 
iso-scan/filename=$isofile noprompt noeject quiet splash
        initrd (loop)/casper/initrd.lz

I really need some help getting started on this.

Thanks in advance
Larry