I've done this (no cardboard option).
did it work?
Afterward I felt like such a hillbilly, the next time hail was predicted, I just parked the car in a parking garage & had the 'I can't believe you're worried about it' GF drive us back to the apartment.
It does work, at least for any common hail I've been in. Larger than marble but just short of the size of an actual golf ball.
Can also set a box of some kind, small bucket, pile of laundry, just more rags, or whatever you can come up with on top the car and fasten the blanket down over that. Creates more cushion or airspace or odd angles between the hail and the car. That handle about anything you're gonna see.
As long as it's fastened down good enough the wind can't blow it off first.