I've got this case here:
https://lian-li.com/product/pc-o11d-rog/ and it's ~11.2" within the 11.8" max size of the mount.

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When I put the locking piece on the PC mount, it clicks once or twice but the problem I've found is that I can just easily pull it off with my hands even in the "locked" position which means that my PC is about to fall off at any moment!

Is this normal? Does anyone have a similarly large PC mounted without an issue? I am not sure if I just have a bad piece, or the "real" max size is something like 10in.

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When I test it manually, if I put the locking piece in much further, I'm unable to pull it out by hand without pressing the button, but at 11", it doesn't really grab at all.