It's a concern. Not a big one IMO in the two pics above.
I dunno what you mean.
In the S550 mount I don't see any problems opening up the hole with that saw and eliminating the two original notches.
This is about the limit of what I'd cut from the tower. Keeping the lip around the opening is a good thing. Could use some clean up.
Some seam welding a bit around the top plate to the tower wouldn't be a bad idea, but not needed.
The main thing to avoid are slots and sharp interior corners for the hole.
This is what I’m doing.
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