How to create a custom Debian unstable boot CD?

[Felix von Leitner <felix-debian@fefe.de>]

I am in the process of creating a Debian boot CD.
Unfortunately, I need to replace the kernel on the CD and I need to add
half a dozen packages (most of them meta packages with lots of
dependencies and a few custom config files).

So I downloaded a Debian unstable CD image with jigdo, and (I am putting
it on a 700 MB medium so there is enough space left) added the packages
and the new kernel, added the new Packages descriptions to
dists/sid/main/binary-i386/Packages and ...Packages.gz, updated the
sizes and md5sums in dists/sid/Release, and burned it to a CD-RW.

When booting from the CD-RW, I get this error message from the installer:

  Drive contains non-Debian CD

What should I do now?  Did I overlook some other md5sum text file I
should have updated?  Does it make a difference if I put the new
packages in contrib instead of main?

Thanks,

Felix