Contact a local canvas shop. They could get you the material to make you an OEM Tumac surlast cover or they could make you a Sunbrella cover. I've had both. My original Tumac cover lasted 12 years. It did its job! Then we had a Sunbrella cover made. In my opinion Sunbrella is an upgraded premium fabric that breathes resulting in vents not being needed. I think you can contact Tumac direct.......https://www.tumacscovers.com/materials
Just had a new custom Sunbrella cover made this week to replace the original 9 or 10 year old Tumacscover. Paid $1000 for it in central South Carolina near Lake Murray. Excellent craftsmanship and every factory snap lines up flawlessly. And the lady did it in just a little more than 24 hours! Dropped the boat at her house 2 PM on Memorial Day and she called me at 5 PM Tuesday to tell me it was ready.
I chose the linen color because the original black cover ruined the tops of the front couch cushions. They are stiff, brittle and cracked where the cover laid on them. I plan to have them repaired at the end of the season.