I pushed my VXGI+VolumetricLighting 4.15 branch on GitHub ( https://github/overfuze/UnrealEngine/tree/VXGI+VL-4.15 ), I am not 100% sure if it will compile for you guys, but you are free to try it 
There is also a build with Flow but when I tried to compile it it gave like 29 errors, so don’t download that I guess.
My friend also made a 4.14.3 VXGI+VolumetricLighting branch ( https://github/1suky1/UnrealEngine/tree/vxgi+volumetrics ), you can try that aswell, I am also not 100% sure if that one will compile.
If you download any of these and compile them, please let me know 