Hardware to attach wood or cement board valence??

I'm installing blackout roller blinds on the exterior side of my very wide 3 section sliding door. They will be inside install (between the brick jambs). I want one roller blind the width of the door panel that slides open, so if someone needs to go outside after the blinds are down, they only need to raise one blind, and the other roller blind the width of the two fixed panels. All that is doable as far as I can tell.

However, I'd like to install my own valence so I can exactly match the paint on the exterior trim, and so that the valence is one piece, the entire width of the 3 section sliding door.

The challenge is that the valence needs to be installed after the roller blinds, so access to the header of the opening will be blocked by the roller blind roll. I can't install something with a slot that the valence drops into, or that I slide the valence into sideways, because then the valence either won't sit tight to the head of the opening or one of the jambs...

Ideally I would like to install a series of some sort of 2 piece hardware where one piece goes into the head, and the second piece is an L shape, where the vertical piece is installed to the back side of the valence material, and then the horizontal part of the second piece slides into the already installed first piece. Does such hardware exist and if so, what is it called and who sells it?