Spacer band for Emerald cut set in knife edge

[autumngems]

I am hoping you all can help, my daughter is looking for a spacer band to put with her emerald cut solitaire, it's set in a knife edge setting. What width and style should we look for? We want something thin but not too thin as she will be getting a diamond band later and doesn't want it to rub the prongs of the solitaire.
I see various sizes and style, flat, domed, etc.
Suggestions?