Well, (if my plan goes to ahem... plan) I'll not be able to have an envelope opening, and I can't to button holes by hand nor with my manual sewing machine... I'm not entirely sure what I'll be doing, I'm not a confident sewer but couldn't see crochet fitting in well with my themes and ideas. I'm hoping when I make my mock pillow the closure solution will just jump out at me
I wonder if twill snap tape would work then? Just a random idea! (I think thats what its called! its twill ribbon with snaps in it. sew one to one side and the other to the other)
I was going to recomend a strip of velcro... I used it a lot way back when I was scared of zippers and button holes.... I even used to sew buttons on to make it look like it really buttoned.....and as canamharris said,... Just a random thought
Or just use snaps without the tape? Or ties? Or button loops? Or loops laced up like you would a shoe? There's also buttonhole elastic...I know it's mostly used in waistbands, but there's lots of other uses for it. Not sure how it would work for something like this... I've also seen button-down shirts salvaged just for the buttonholed and button parts. Can save so much time.
Just some more random thoughts.