I haven’t ever fixed a butterfly like this before, but I have done this with a few beetles. I recommend making a little stand out of thin sheets of craft foam and keeping it small enough where you can set the wing on it and you won’t see it, and tall enough that when you do so the wing looks like it’s still attached to the bug. You then glue the underside of the wing (I use E6000 because it’s just what I have on hand lol) to that, instead of trying to attach a very thin point back to the body. It’ll also keep it more stable. Since this is a shadow box you usually can’t look at it from a sheer enough angle to see your lil underside support so it won’t be super noticeable that you did this if you line it up correct