Well, finally I got it right and managed to download the right version of CD #5.
There was a twist in the tale, though; after downloading the ISO image, I found that I had run out of blank CDs. After a bit of head-scratching, I figured out the solution: mount all my five ISO files and make YAST use the resulting file system as an installation source.
Result: I've finally, yes, finally, managed to compile and install the Crystal window decoration. The effort was worth every bit of the struggle. Check out this screenshot.
An unintended benefit of permanently mounting the CD images in /etc/fstab is that my system has become very snappy as well, for whatever reason. Plus, installation of additional packages from the hard disk is, of course, much faster.