File Prep For UV Print on Black Base

Hello

Man I am struggling with this. So attached is an image I need to print on black. My UV printer does white, but the gradient is causing me some concern. It was suggested to do two files, my CMYK file, and a white base file, the issue though, is how do i extract just the white when the opcaity is mid range on the black? I can create a spot channel for white, but the original art contains a glow, and when I select just the art in the layer, even with the tolerance at 1, it selects the whole aura of the glow. That will cause my spot channel to print white under the entire marquee. Do I need to flatten the image and extract all black? And then make the remaining pixels my white base? Im so lost on this. Thanks for any help. On the right you can see how the file was originally created. The glow is around the chest, above the body and legs and throughout the hair.