Is there an alternative to mkusb to install Ubuntu on a portable SSD that works with both UEFI and BIOS boot methods?

I’ve successfully used mkusb to install a persistent Ubuntu 18.04.2 LTS system on a new Samsung T5 USB SSD. What is nice about mksusb is that it creates a bootable system that works on a wide variety of computer systems. However, I would like to have a portable ‘real’ Ubuntu installation vs. a live/persistent install that works with both UEFI and BIOS boot methods.

The process outlined here looked like it would do the trick, but in my case I’m just presented with the grub prompt when booting the drive. Note, that I interpreted the step:

Cut grub.cfg from sdx5/boot/grub and paste to sdx3/boot/grub, overwriting the existing grub.cfg file 

…to mean that I should move the new grub.cfg file from /dev/sdx5/boot/grub to /dev/sdx3/boot/grub, overwriting the grub.cfg file created by mkusb and deleting the grub configuration file created by the installation from the installation partition. Also, /dev/sdx3 was not mounted after the Ubuntu installation completed, I had to manually mount it (/dev/sdx5 was already mounted on /target).

Since mkusb without modifications works fine with the T5 SSD drive, I assume that there is something about the Ubuntu 18.04.2 LTS installation grub.cfg file that is causing an issue.

Is there an alternative approach that would work, or is there something inherently different with a portable SSD vs. a USB thumb drive that prevents creating a truly portable drive?