Should be a straight shot, between the rafters.
I would use a stud finder to locate studs in the ceiling. Then I'd cut a single gang hole, and use the fish stix/glow rods through that hole, aimed toward the knee wall. The rods should deflect downward when they hit the ceiling. I would definitely use fish stix, to start with, rather than a steel fish tape.
Why do you think the fish stix will not bend down the wall?
Cut your hole as far away as possible from an adjacent stud, i.e. you want to angle the rod low, if possible, and avoid a vertical position. As long as you hit the roof at less than 90 degrees, it will bend down toward the knee wall.
If the roof is very steep, maybe the fish tape would be better. Or, if you're good, you can stab the rod up from the knee wall space, and catch it from above - but you'll need to calculate carefully to make sure you're in the same stud bay area. You can grab the rod above the ceiling with another rod from your hole, with a hook on the end.
You have to make certain assumptions about how the house was constructed.
If you can pass a rod up to the ceiling area from the knee wall attic, before you cut the hole, you'll be ahead of the game.
Another set of Harbor Freight rods (cheap, never used them myself) may help. I don't know how far it will be.