The thread plate is now mounted to the base with thread lock in select locations. The top can still come off easily so I can drill holes to mount the gantry to the alloy tongue that comes out the bottom middle (there is one on the other side too).
Without the 75mm by 50mm by 1/4 inch 6061 alloy angle brackets you could flex the steel in the middle. Now, well... it is not so easy for a human to apply enough force to do it. The thread plate is only supported by 4 colonnades at the left and right side. The middle is unsupported to allow the gantry to travel 950mm along. I think the next build will be more a vertical mill style than sliding gantry to avoid these rigidity challenges.