I suggest rubber grommets. You can get them at auto parts stores. They are used to protect electrical wire from being cut when running through sheet metal. You can probably find them at any stereo install shop as well. You just push then in the hole and a rubber lip surrounds both sides.
If those are too heavy just use heavy wire and or even a cotter pin and loop it through a hole and then tie to that. Maybe even try searching for nylon arrow clips. These are plastic plugs that have a loop body. You could push one through a hole and run your cord through it…


