I like your idea of misting the canvas with water and ironing.....
I zip mine together to see if they are saggy....they are in the middle a bit and I'm not sure if it's because they are stretched or if I sewed the zippers on a bit wonky.
If I zip them together and mist them with water and blow dry them, do you think they will shrink up or will I just be making things worse?
I also like the idea of marking the canvas.........I often find that the bottom border is more stretched than the top border even though I measure when I pin them to the zippers.
Thank you for the great explanation you gave to Joy.......!