It's not clear... Do you want a portable USB drive ? If that's the case it's easily doable with Arch or Fedora.
If you want a portable USB that you can modify AND flash then... It's a little more complicated. You can always make a bootable Arch USB then rsync in any existing drive but it seems a little complicated.
What you might want to do is create a simple install script. You can pretty much do it for any distro. It will consume more bandwidth than copying/writing an existing distro but will prevent MANY errors.
With Arch it's quite simple. I believe it might be as simple with Debian or any other distro.
Steal