When I think "night guard", I think of it in terms of tooth-grinding at night, & trying to protect the teeth from that wear.
When I think "worried about my teeth shifting", I think of "retainers", which to me is a different thing.
I'm no expert, but from my experience, if you need the former to keep teeth from grinding against each other at night, I think the cheap ones work just as effectively, but they do not keep teeth from shifting. For the latter, I would want good retainers made by a dentist. I had invisalign, & I can tell when I've had the forms off for a while, or now, the longer I go without retainers, it almost feels like my teeth are already starting to shift until I wear them again for a while.
In my experience, regular "night guards" did not keep my teeth from shifting over time.