I would start by tying a couple of standard "loop" knots with doubled-up leader if it were me. Examples are the perfection loop, or the surgeon's knot - only double the line first.
I don't feel the need, so I never needed this, but if I did, that would be my starting point. I would check "break strength" with a scale at home beforehand, however.
Just another thought - if the knot is generally recognized as the weakest point of the leader, does it increase strength any if you go from a single strand to a double strand with a knot? Not being smart, just asking.
(If I still was working, and had lab access, I would do some testing and give you the answer!)