For what it's worth, when I first started out, I was trained to use Fusion to mount touristy ones. We would use a self-trim method to apply the Fusion. In your press, you put in order from bottom to top, kraft paper, the papyrus face down, Fusion, release paper. Cook it for a full cycle, then pull it out while it's still hot and the Fusion is still melted. Quickly pull the papyrus from the kraft paper leaving the melted Fusion stuck to the kraft paper. It'll trim itself to the shape of the papyrus. You then have your papyrus with adhesive on the back trimmed exactly which you can mount on whatever matboard you like.
Nowadays I'd almost always use starch paste and mulberry hinges, but it's a handy tool to keep in your back pocket for unusually shaped items that need to be flat.
James