After discovering the flex in my Z axis, I decided to see whether the flex was present just by moving the X-axis. I wrote the following snippet of gcode:

G28 X0
G1 X250 F3600
G1 X10  F3600
...repeat the above 2 lines 10 more times...

and this is what I saw (with the original MTW mounts):

I decided to design a brace, which looks like this:

It’s designed to fit under the existing mounting plate:

And installs like this:

Now I get virtually no deflection:

About a day after I designed this, the MTW folks came up with a much simpler, easier/faster to print version:

These perform almost as good as mine, but are easier to install. Here’s a video showing the deflection while moving the X gantry back and forth (sorry - it’s slightly blurry):

I decided to use the MTW version due to end switch interference with my mount.

To summarize:

• 0.1mm deflection using the MTW mount that came with the printer.
• 0.03mm deflection using the modified OpenBuild mount.
• 0.01mm deflection using the MTW mount with the printed MTW braces.
• 0.005mm deflection using the MTW mount with my heavy duty brace.

The MTW braces are available here.