When I ship 5.25" floppy drives, I cut an insert from the top of a
large yogurt (or Cool-Whip, if your dietary preferences lean that
way) lid and insert it into the drive. It doesn't shed abrasive
material and is soft enough to cushion any head impact. I have a bit
of a concern about cardboard being hygroscopic and leading to grief
somewhere down the line.
I've also got a few plastic 3.5" dummies that were shipped with new
drives. No manufacturer given; just "Made in Japan" and "Insert this
sheet under transportation" (sic) in several languages.
--Chuck