Not sure if MNT Has any policies on that, but I think a toploader packaged correctly should be fine. And you stated, it should keep it from floating around If the correct toploader size is used.
Here' MNT's site if you need it:
https://mntgrading.com/
As a side note, I like how this whole grading increase has given MNT more exposure. Always liked having another alternative to the other big guys. Being from Canada is a nice plus, too.
Best of luck!
