Same… tried rocks, Bombs Away! neither counted the multiple times I succeeded in 4 consecutives.
From what I can tell it can only be done suing unmodified boxes which… requires you to not spec your skill tree past the grenades which is just dumb and I assume not intended.
Personally I refuse to do this because being on heresy+, I don’t want to be deadweight to my team because of a stupid penance. (if it was Malice or less, maybe?)
Hoping they allow variations of boxes to count, even rocks is still a somewhat challenge (less so because of not having to hog grenade ammo from the entire team to do it…also a dumb motivation)
i did it same day i posted this by not speccing my tree like you said. It honestly wasnt that bad, but for most people i can see why they would never bother doing that as it can be a hassle.