Lots of pleasure boats have removable canvas covers, side curtains etc. Marine canvas places are familiar with working on pieces that are unusual shapes. You might find help with a marine or boat canvas shop. Keep in mind that anyone with a heavy duty sewing machine can call themselves a canvas shop. When I had a motor yacht I used 2 local shops that were one person, low overhead places that did excellent work for very reasonable prices. The individuals skills are what makes the difference. Ask locals questions and shop around.