Hey, Ross. I fixed my topping lift today. Used 3/8" line that replaced 1/2" line. Here is what I did, but I don't know the name of the knot I used.
On the boom end I used a bowline. Ran the line up and through the loop in the topping lift. Came back down to the bowline, put a loop in the line then wrapped the bitter end twice around all three lines. Passed the bitter end through the just made loop near the bowline and tightened. The knot slips but only when I really push it. So I don't think it will slip on its own. I found the knot to be called a whipping loop.
Tada!