Download Hiren's Boot CD and run one of the many erase programs on that. Dban is good, set it, others car good too, just make sure you use a random sequence over three passes - any other combination is predictable and therefore possibly reversible. Dban won't save a file to anything except the floppy, and a CD is read only so should be fine.
I also made a DOS 6.22 floppy that uses SecureErase
that worked well, the project was once sponsored by US govt's NSA so should be good! It didn't work on everything but seemed quicker than most of the others.