Using a mask to apply a subsurface only on a part of an object

So, I did a lamp (the materials will change, it’s just for a test x) ) and I wanted to apply a subsurface on the lampshade, but only on the red part (btw, there’s no thickness to the lampshade, as we’ll never see below it, don’t know if it has its importance)

It’s been a while I didn’t touch to the materials in UE4, and I’m a bit lost. I have a mask with everything white except the corresponding UVs, but whatever I do, it always apply the subsurface on the whole object (or at least, doesn’t give me what I want). Most of the time, it gives me that (with a simple subsurface) :

I try different combinations, changing the smapler type of the mask, unchecking sRGB, using different channels,… but nothing worked. I know I’ll have to separate the lampshade from the base (at the moment, it’s one object, but if I don’t do that, the base will not cast shadows), but there’s still the golden part of the lampshade that will be a problem. I searched on internet, but didn’t really find anything about that (or I’m blind, that’s another idea)
So, if anybody have an answer for a lost boy, I take it x) And if you see I’m totally wrong on how to do a subsurface effect on a lamp, you can tell me too x)

Thanks in advance !