Question on a latin phrase (Magna Opera)?

To be vague, I'm ordering something custom that I can personalize. It's rather expensive.

Now, I initially decided to have Magnum Opus engraved onto it, but I didn't want to discredit this person's other pieces by saying that everything else he made wasn't "a great work'.

I then thought about the pluralization, Magna Opera, and thought that it might work. But I don't really know latin so I'm a bit lost. I want to make the piece seem important but not the pinnacle of the world. I would like it to be A masterpiece not THE masterpiece. It is one of the masterpieces to clarify.

So from here on I'm a bit confused. Does Magna Opera work as a substitute or does it not make sense? I don't want to have "Masterpieces" engraved because that doesn't really make sense.