Nice work !Its pretty much a "u"-shape I found at a hardware store which I cut to the correct length, below is another picture with it unpainted, should clarify it a bit I hope.
The part I attached to the "floor" of the 5.25 drive bay area is cut from a 1.5mm aluminium sheet. Its not super nice and straight since I had to form fit it by hand, but black paint covers a lot. And that area is covered by the side panel anyways, so only visible when the case is open.
Curious about a couple of things, why you made the 5.25 part rather thank cut the stock 5.25 side panel and mount it and do you feel you have enough area around the base for bottom exhaust to flow freely away from case?