To add some clarity, the channel base metal is brass that has been chrome plated. I am thinking I need to remove the plating down to bare brass at each joint.
Then join them with a process that will accept chrome and have the revised trim replated.
(The factory corners definitely appear to have been brazed.)
I plan to build one long piece to cover the top edge and bend and join the pieces necessary to form a U-shaped portion to cover the other three sides.