Making a live bootable usb is not easy not sure if possible. What I do when I have built my system is to.
1 Shrink the partition as small as it will go ( I use gparted from a live cd)
After shrinking you can resize partition back.
2 I then use the DD command to copy the partition to where ever I want to save it And compress it with Gzip.
I can then if I need to just Unzip, DD back to any partition.
If you want to put it on another machine the problem is
The architecture 32 bit or 64 bit, and different drivers setup in Kernel. Also grub.cfg and fstab needs to be edited, if partition setup is different.
Think I have covered all