Here's a question for you all. I've got a map I'm in the middle of (actually almost done) that needs a legend. Now the legend should go in the sea where there are no details. It's a big poster map so we're not short of space. But I can't figure out how to put in a legend that fits the style and looks good.

The map is a world map and the title is in a cartouche with a metallic style. I don't want to compete with that. It's all in a style very similar to my regional map from a while ago:

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Now I have a frame in mind for the legend, and the overall map has a very plain double line border. However if I drop a black frame onto the dark sea it has a good chance of just disappearing. Also, if you place the frame down, what happens inside the frame - is it also sea still, with the legend text in white perhaps? Should it go back to bare parchment? That's going to stand out a lot and look a little strange?

Any thoughts much appreciated!