The clamp is installed backwards if it matters to your inspector
I know this comes a day late and a dollar short, but the screws go on the outside, and the nut goes on the inside.
Additionally, that clamp cannot be used on a plastic box, because there is no provision to ground it.
I would not consider modification of a cheap plastic box, but the one in question is the much stronger fiber reinforced kind. I think that drilling it for a cable clamp might be ok, but only if you can find a plastic clamp that suits your needs, or if you can find a metal clamp that can be grounded.
But kudos on switching to metal boxes in the end. Good choice.