The center of my heat bed was lower than the rest - this took care of that.
Use this to generate a center bed support model of any height which mounts between the y-carriage and bed, using the mount points of the y-belt holder.
You can then adjust the height at the four corners of the bed to raise or lower the center support relative to the corners.
Ironically, I have no idea why only the height setting doesn't work in the customizer web app. It works fine in OpenSCAD. Maybe someone can tell me what I'm doing wrong here :)
Works well with a hard foam spacer or a few layers of cardboard to insulate the part from the heat if using a heated bed. Adjust height accordingly.
CAUTION: Using this will fix the center of your bed in place, so a head crash will be much worse. Use at your own risk.
Example values for my Anet A8 stock y carriage, using a 12.5 mm thick, 30 mm x 30 mm hard foam spacer.
** The foam covered a thermistor on my heat bed, however I didn't notice any heating issues after installing.