The solution I would use:
Go to a sheet metal worker and manufacture a flashing plate 2 degrees steeper than the roof pitch with an insertion edge.
Cut a 5 mm slot with a circular saw/plunge saw in the panel.
Apply a bead of high-quality exterior sealant, no cheap stuff.
Insert the metal into the slot and secure it with screws.
If you're going to replace the panel anyway, I would probably nail 2-3 28*70 gles on the wall along the ceiling and then nail a stånskiva onto that and then nail the panel on top. Leave 3 cm between the panel and the plåten. Don't forget to saw a droppnäsa on the panel but not on the battens.
Do as hantverkare1 suggests. You can go ahead and nail down the cover battens on top of the sheet metal, but stop a bit above the bottom and saw drip noses. The connection to the roof should also be sealed, and you could apply some sealant at the bottom edge or alternatively use a rubber strip that seals against the metal. Otherwise, there is a risk that water will run in behind if the roof doesn't slope away from the metal. See sketch:
But if you're going to replace the panel, there's no reason to saw grooves in the panel and insert sheet metal there, right? You do it like building instructor Bob writes above. Similar to the way sheet metal is placed above a window.
