I've grafted them with inverted cleft graft and had success with the luc's garcinia scion stems being slightly larger than the rootstock. It appears I centered the stems although I guess the safer bet is to align one sides cambium. I wonder if anyone has tried a inverted veneer graft because that seems like it would be the solution for have a scion that is too thick. Or maybe try cutting out more inner material in the V groove for an inverted cleft graft and thus making the cutaway profile slimmer.
I mean how young are you seedlings, like just sprouted this year?