I'm a MegaMan Battle Network nerd, so I did something with that.
This repository contains a vanilla PET overlay, a MegaMan "edition" PET overlay, and for those who like the Gameboy Advance SP and/or Gameboy micro overlays, there's a little somethin' special for you too.
NOTE: You can put these wherever you want, so long as you are able to easily navigate to them within retroarch's settings and so long as they're not with any sensitive files that are crucial to retroarch or your Anbernic device. Otherwise, it may be best to use my recommended defaults locations.
Move the OtterBot folder to mmc > MUOS > retorach > overlays
Open a GBA game, then the Quick Menu, and go back to the main menu. Then go to Settings > Video > Scaling.
Integer Scale: Off
Aspect Ratio: Custom
Custom Aspect Ratio (X Position): 0
Custom Aspect Ratio (Y Position): 0
Custom Aspect Ratio (Width): 640
Custom Aspect Ratio (Height): 426
Bilinear Filtering: OFF
Crop Overscan: ON
While playing a game, open the quick menu and go to On-Screen Overlay > Overlay Preset.
Select Parent Directory until you get to the directory that contains the folder named OtterBot. Then go to OtterBot > gbasp > PET or OtterBot > micro > PET and pick the overlay you'd prefer.
Go back to the Quick Menu and then go to Overrides.
If you want the selected bezel for all GBA games, select Save Content Directory Overrides.
If you want the selected bezel for just the current GBA game, select Save Game Overrides.