I made a 6" high riser, open front; 2 ends and back. Put 1/2" thick glass on the riser (local glass shop), TV on the glass. I had matching wood, you could simply paint it matte black, or stain if you are good at matching. Or, pre-finished shelving, bottom and rear cuts won't show.
I put my horizontal Klipsch Center Channel in the space below the glass shelf. Get one with no rear port(s).
NECK PAIN: Donna is 5’, I’m 6’. We made temporary stacks of books under the TV to test the angle of your neck while watching, often a few hours. That gave us the height of the riser. Now verify you have enough height for your center channel (including future change.
New version of ours
https://www.klipsch.com/products/r-30c-center-channel-speaker
Ours, new
I bought used, only the front shows below the glass shelf, black is easy to touch up
REFLECTIONS: Ours didn’t change, but when making temporary stacks of books to establish height, check the reflections, see if something becomes problematic.
Our Klipsch center channel has a black fabric front that is magnetically on or removed. You might want to verify that.
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btw, Home Depot will cut boards for you, both width (height of riser), and either you or they cut the lengths, less critical as only rear corners. Couple of rear corner angles or glue blocks, ....