Heater Box / Cowl Seal

I don't recall mine having a solid side. All foam, about 3/4 square. I do recall using same 3M weatherstrip adhesive on it. I would think your solid side goes to the box which is likely the flattest surface. Then foam side could conform to the cowl.
If you look at my pix, the solid side is showing.. The sides of the seal that are pressed against the cowl and box looks like foam.