The canvas sling had excessive stretching causing the sling to be too close to the elevator push-pull tube when a passenger is in the seat. I know I can't be the first to experience this.
I did a lot of research and found acrylic material is a superb material to minimum stretching, tearing and general wear. I used 9.5 ounce acrylic material for the sling and had 2" seat belt material sewn on to the edges of the sling lengthwise. I have the seat belt material sewn into the seat back portion of the seat in a "V" from the center of the "floating" tube to the top outer corners of the seat back. I also have seat belt sewn into the center lengthwise along the horizontal portion of the seat. From the front the seat belt then forms a "Y" and has seat belt along the edges lengthwise. I had the pocket enlarged for the "floating" tube to accept a 3/4" tube to prevent bending of the tube. I also skipped the metal grommets on top of the sling and I formed a sleeve so the top bar slides through the sling just like the bottom hard mounted tube. I have my rear seat sewn exactly as above and had 300 lbs in the rear seat. It keeps the seat 2"+ above the elevator push-pull tube.
I purchased my material from a company called Rochford wholesale supply out of Minneapolis. They kept popping up during my researching on the internet. They were very helpful, friendly and the prices are great. They sold the acrylic for $11.00 a yard and the belt material for $.48 a yard. The seat as above was made with $15 in materials. If anyone wants a more detailed description on this seat don't hesitate to write. It's tough as nails and works great.