I don't have a length either, but when I replaced mine, I used a new seat cushion support web. The material is the same as original and is slightly elastic, being made of rubberized cloth. The part number is ... 281-858
If you make them to short, the top will be hard to latch because the latch location pegs won't line up well due to the frame being deformed. What I did was to almost latch the top frame and fit them, then latch the top and check the tension, which was good. They don't need to be drum tight, just not loose.
Take what I say with a grain of salt. When I drive mine at speed with the top up and the windows down, mine flutter and make quite a racket, so I guess my tension is wrong, it may be too tight.
-Michael