You don't need a soft shackle. You don't even need a split end to your mainsheet.

On the back of my rear beam there are two eyelets. I attach a small loop of dyneema on one side. I then thread the mainsheet through the traveller, through the loop, pass it inside the other eyelet and make a figure of eight knot. Done.

The first time you do it you need to play a bit with the loop so it is the right length to center the traveller, but then you leave it on board permanently and you don't have to fiddle with anything to rig.