What connectors are folks using within J Boxes for the transition ...to THWN ?

While it doesn’t answer your question, I’m planning on using my two j-boxes (one for each string) as a “pass through” to a combiner box in the attic. To make that feasible I ordered two 50’ male/female extension cables, then cut them cut in half leaving a total of four 25’ cables that are wired together within the combiner box.