Our trunk corner metal is formed in a brake using pre-finished aluminum.  It’s 2″ wide on each side, 4″ in total.   Sold in pieces that are either 28″ or 36″ long each.

Powder-coated black with a matte finish; you can wipe on tung oil to get a glossier finish if you’d like it to look that way.
This is thin gauge metal; fairly easy to punch your nail holes through before you start attaching it to the trunk.

One-piece, 28″ long, 2×2 inch, with shipping to any USA address, is $15. A set of 4 pieces, 28″ each, is $55, 48-state USA shipping included.
One-piece, 36″ long, with shipping to any USA address, is $20. A set of 4 pieces, 36″ each, is $75, 48-state USA shipping included.