I am in the home stretch restoring a 1929 Murry Town Sedan. At the 9th hour, it was decided by the new owner that he wanted a lap robe rope installed on the back of the front seat carriage - which I had already covered with material. This presented a problem because I could find neither a lap robe rope nor the brackets in catalogs or by advertising on-line. Sammy Guthrie of Arizona Model A came up with a nice pair of brackets that match the assist rope brackets near the rear quarter windows. That was half the battle.
I bought a length of thick twisted rope and picked out some material from our local Hobby Lobby and took them to an upholsterer to mate to each other. He suggested instead of rope - whose curls would always be visible through the covering material, besides shredding - that we instead use a solid rubber core. He also nixed using the not-too-matching material I picked out to cover the rope. He pulled out a sample material book and recommended a color with a pattern that complimented the cloth seatback covering. He ordered the material and rubber core and sewed the material around the core. Only a little selvage sticks out past the stitching, which I turned towards the seat back so that it wouldn't show. After flipping the front seat backrest cushion up, I drilled holes for the brackets and installed bracket bolts and wide flat washers with nuts on the inside of the seat carriage. The new lap robe rope was stretched between the brackets.
I think it turned out well, all things and sleepless nights worrying about how to do this considered. Check with a local upholsterer and see if he/she/it/they can come up with something similar.
Marshall