it fits in the mtl pod, but the thing is there are two versions of it. the one that has only one o ring at the bottom apparently doesn’t fit any of the pods. you need the one with two red o rings at the bottom. i got it from sourcemore, couldn’t find it in the uk.
basically the o ring at the top fits round the outside of the part that the smaller stock coils fits on the inside.
the voopoo pnp rba also fits. it’s a bit easier to build but the GTX one is better, and the voopoo one also seemed to be threatening to leak the couple of times i’ve used it.