The way I'm doing it on my canoe, is the same only a bit different. I normally sit in the center seat to paddle. The center, of course, is the balance point for portaging. So, my permanent thwarts will be tied in just fore and aft of my seating position.
The "thwart" that will be used for portaging is a nylon strap about 3" wide, sewn into a loop that is almost as wide as the beam of the boat at the center-point. It will be tied permanently on one end, and only temporarily for portaging at the other end. This nylon strap will be what rests on my back for portaging.
Tumpline will likely be two pieces of cord tied to the corners of a bandanna that has been folded into a triangle, then rolled. The bandanna will go over the front of my head. Stays put, soaks up sweat, cheap, can be used elsewhere if needed. The two cords will be tied to the portage thwart a few inches inboard of the gunnels.