@tdelios you can find it here :
https://github.com/recalbox/recalbox-os/wiki/Configuration-recalbox-pour-Bartop-Born-Arcade-(FR)#d---configuration-du-menu 🙂

I had totally forgotten about overlays - thanks acris! 🙂

Going to give it a go when I get a chance.
Think it will be easier to do a single system first due to every game on it being the same resolution etc.