I am making mine. I found great free clipart online and I'm just printing pages on cardstock. I am making mine about 5x7" because I like sticking the photo from the M&G on the side opposite the autograph (so standard 4x6" photos will fit). Instead of spiral binding, I prefer to just punch a couple of holes and use book binder rings. It's cheaper, and it's super easy to open the book flat for autographs (makes it easy for the characters to hold) and if we end up missing some of the characters, I can just remove those pages when we get home, or the kids can move the pages around if they want to change the order. I can also add extra pages afterward for additional photos or mementos.
We use the retractable sharpies for signatures. If you are using them as well, just make sure that your pages are thick enough to not bleed through.
And if you are looking for an even simpler idea, you could just cut cardstock to the size you want, get the binder rings and buy some Disney stickers - let the kids decorate their own books!
Good luck and have a great trip!