There are many resources on the web and in print of on how to remaster Knoppix 3.x. Here are some notes on the differences between remastering the 3.x series and the 4.0 release.
* If chrooting into the new customisation fails with many "bash: /dev/null: Permission denied" errors, the problem is that the partition you're working in has been mounted with the wrong args in the host, you need the ... -o dev ... option. This has changed from the 3.x series, apparently because the default partition mounting options have changed with the move to unionfs.
* If the CD image is much larger than you expected, the problem is likely to be that you've forgotten to "umount /proc" before you leave the chroot, and so the image contains random chunks of RAM.
* updatedb appears to be broken (for me anyway), due to an issue with /etc/mtab and /proc. The solution appears to be to clear /etc/mtab after chrooting, then mount /proc, umount /proc, then do updatedb
* After moving to a more modern machine, k3b was quite happy to burn knoppix CDs at 40x. Unfortunately they weren't bootable due to data errors. Reducing the writing speed to 12x results in consistently bootable CDs.
The forums at http://www.knoppix.net/ are also great sources of information on remastering knoppix