I understand your "solution", a VM and so on, but that's not what I can use/need

I need to run Knoppix on very old machines that are not mine and that can't boot from USB key (old Bios)
so I have to use a DVD for Knoppix (except if sometimes they already have Debian on HDD for exemple, but I can not modify them) + a key for my data (and my /small/ softwares stored here are protected from any unwanted modification)
-- while til now, default Knoppix 7 scripts allow install of a crypted persistent container only on the boot device

the only "new" solution you give me is remastering, that I wanted to avoid if possible