What more to associate with Valentine's Day than a heart and some rings. Maybe a flower :)
Anyway, I chose the heart. Like a simple box or a ring case. The two symmetrical parts are connected by magnets, then the ring can (but doesn't have to) lie on the backing that is on the other plate. If I could print the soft bed that is used for rings, that would be better, of course, but we have to deal with what we can print, so I chose that route. 
There's a hint of guide grooves made for the backing. Enough to assemble the two parts when you insert them, but not enough to prevent the girl from removing the ring.
The case has an enlarged space in the area of the possible stone, so you should be able to fit a possible multi-carat stone in there ;-) 
As you can see from the attached photo, the flattened part of the backing is supposed to be under the part of the ring with the stone, and it also shows where it goes when assembled.

I personally used my favourite D4x6mm magnets for the construction, but BambuLab doesn't have such magnets in their offer, this can be easily solved by inserting a pair of D4x2 magnets instead.

When I created the box I had some wood filament in mind, but after printing when I used red PLA... I think this is the perfect place to use PLA Crimson Red Sparkle

If you like my work, you can buy me a coffee