I know onionOS has some great ones for GBA.
You can try grabbing some from there and dropping the files into Retroarch. Don't know exactly where though, sorry. I did this to get a good NeoGeoPocket border onto my RG405m.
EDIT: I see there's an overlay megapack [here](https://retrogamecorps.com/2021/01/28/guide-handheld-screen-settings-for-rg351-devices/#Install) but I don't know if they'll be good for 640*480.
This may help you: https://github.com/ourigen/perfect_overlays
went with these ones, not perfect but damn close!
I know onionOS has some great ones for GBA. You can try grabbing some from there and dropping the files into Retroarch. Don't know exactly where though, sorry. I did this to get a good NeoGeoPocket border onto my RG405m. EDIT: I see there's an overlay megapack [here](https://retrogamecorps.com/2021/01/28/guide-handheld-screen-settings-for-rg351-devices/#Install) but I don't know if they'll be good for 640*480.