The absolute safest way IMO is to not use a leader at all. Just thread a bead onto your mainline and set it at the depth you want the rig to slide back, then put your rig on, then another bead, then a 2 inch length of anti tangle tubing to protect the mainline, then a 1 inch length of silicone sleeving, tie a quick link onto the end of your line via an over hand loop, attach a lead and slide everything but the top bead down to the lead.
It's best to use a sinking mainline and make sure to mould putty around the base of the swivel on your rig to stop it floating off the deck.
I hope that makes sense ??