Re Ryumbeke's method: I just tried running a virtual machine (WinXP) from a nominally very fast USB3 stick under remastered 6.7.1, but I had to use a hard disk drive instead to get it to work satisfactorily. Running Knoppix from the stick went very well, however. So, external SSD USB3 mounted is probably the most robust way to do it.
He seems to use the "cloop chain" method (can of course also be used with squashfs), also mentioned elsewhere recently, and I think having a completely pristine OS to boot (as an alternative which Knoppix always allows) may be a good idea. As there is nothing particularly worthful in the original Knoppix cloop, I personally prefer recreating the cloop image. For example, just the deb package database takes around 400 MB in the DVD image, and that is mostly dead space after performing some extra installs. Likewise, programs may be mostly dead after upgrading - therefore, I may easily end up with, say, 1-2 GB of unnecessary occupied space if I don't recreate the cloop.