It’s a flat sheet of soft .032-ish aluminum that’s easy to make and difficult to install. No one sees it and no one cares, so don’t obsess over perfect aesthetic. All it does is cover insulation and prevent rodent access. David’s example is absolutely correct.
You’ll want to remove the rubrail and pry the outer skin away enough to slip the tabs between it and the u-channel. You may find obstructing rivets that you'll chisel slice through with no consequence.
The existingcover for your waterheater hole will need removal and redesign. It shouldn’t rivet outside the rubrail as shown in your image. The bellyskin wraps up onto the floor in the waterheater hole.
You can drill and set skins with pop rivets into the U channel, and additionally securely sandwich it all with the rubrail. Measure for correct rivet length.
I've never seen two waterheater panel repair/replacements done the same way. Everyone figures out their own path.
Here's the original 1968 Bowen waterheater cover, and how the belly wraps into the hole.