If you were to install a wall mount stand for the center speaker above the TV, I think it would make more sense to move the TV wall mount higher. It would be about the same amount of effort. I think the center above the TV would be too high. It can work and I have seen plenty of photos with the center above the TV, but ideally you want the center speaker to be close to the same height as the main speakers. Otherwise dialogue that moves from left to center to right will also move up and down. It depends on how far away you sit from the speakers. If you are 4 or 5 meters away, then the height difference is not so noticeable. If only 3 meters away, then the speakers are more localized.
If you can not move the TV higher, then an AV rack with the speaker on top, just below the TV, should work fine. If the center speaker is aimed too low (at your knees) then it helps to angle the speaker upward. You can purchase adjustable stands but even something simple like this foam wedge will work (and is a lot less expensive).
This is how I have my speakers arranged.
I replaced the old plasma TV with a larger OLED last year, so I replaced the wall mount and moved the TV higher. I chose that height for the TV so that if I lay down on the couch, the center speaker does not block the screen. That is not something you would normally think about when installing a TV, but if I had mounted the TV lower and then later learned that the screen was blocked when laying down, it would have been very annoying.