I would like to see what you come up with. A few days ago I emailed Ron at MLD about retrofitting a head slot to my Spirit 38. He said it wouldn’t be a problem but that I would have to wait until after Summer. If DIY is easy enough I may just go that route so that I can use it this Summer.
I wouldn’t think your materials would be an issue. The Spirit uses 10d fabric and comes in a 50° version with a head slot.
I like the giant buttonhole idea. I would think that you would also have to bind the edges of the slot with a grosgrain or something similar. That would prevent the corners of the slit from tearing and provide a good base for the closure, i.e.: snaps, velcro, buttons, etc.