upholstrey help

When I got my Duster the interior had been redone. When I sit in the car I feel like I am sitting way too low. My original Duster back in 71 I sat a full 4 inches higher.
So yesterday I turn over the front bench seat to see why. I did notice that the springs were sitting there all by their lonesome. I figure to help support the weight the springs needed to be tied together so they can help each other.
I took a handful of tie wraps and tied the springs together, each in 3 places next to the springs on either side of them. It seems to work great. I am sitting at least 3 inches higher. The only problem is in this climate tie wraps will not last long under that pressure. I need some ideas to tie the springs together with a non-metallic fastener.
Anyone have any ideas???

Ed