edited out
Is there a drain hole in the bottom? If so, stick a pin in with five rounds, measure, then with six rounds, measure, and put your pin halfway between?
But I don't know about doing it in Canada....I think technically you could get in some trouble,I know a guy has even been busted for importing just mag bodies.... Not that it really matters in practice, but I'd personally not ask about it on here as it incriminate you for owning prohibited devices.
Be careful