They are supposedly made of Mylar, and some packing straps are really, really close in appearance. Just the other day at the office, I noticed this when I compared the straps of the RL60.2's I have there to the straps that tied a bunch of unfolded packing cartons together. Unfortunately the latter are only 5 mm wide whereas the RL straps are more like 10 mm, otherwise I would've sent you a length of that!
I might be worth it to ask the dealer anyway, but these speakers are getting so old that the chances of getting original spares anymore is rather slim.