How to create mtree files?

[freebsd-questions at auscert.org.au]

Hi,

I would like to use the mtree utility to confirm no changes have
occurred to system files since 'make installworld', similar to that
possible with 'mtree -f /cdrom/5.4-RELEASE/base/base.mtree' on a
release installation. This would also give the added advantage of
being able to determine the current buildlevel of an installed
system, I believe.  Is it possible/easy to create new mtree files
after the buildworld or installworld process? I've looked at the
Release Engineering docs but it seems more than what I'd like to
do. I should add that I am running tripwire, but I really want to
have a quick way to verify which files were part of which installworld.

thanks,
joel