custom Super GameBoy border overlays
Glitched SGB Border #9, IllusionRecreated by hand, based on this video.
Hover your mouse over the titles for preview & download.
All of these can be used with the emulator BGB, and most likely also Gambatte inside BizHawk.
You could turn them into RetroArch overlays if you wanted, but that's relatively harder to do than just clicking a few buttons in a settings menu.
The wider ones are designed for use with 3DS Virtual Console injects, which is in turn the reason why they have a worse overlay to actual screen ratio.
Now that you're here, driven by mild curiosity and boredom I imagine, mind telling me what your favorite GBC game is? If you have one, of course.
The poll is completely anonymous, by the way.