What did you fill it with?

You might want to keep a regular inspection of the tubes where they attach to the chainplate just in front of the rudder .

Two comments.

1.) if you see you are tightening the backstays often (there is a problem) inspect right away.

2.) If the rig starts to fail, it will show up as the square part starts to become round (could give you time to change direction and unload the rig?  This could be good if you are in the cockpit.  i.e the failure will be slow at first, not just a quick pop of the stay, others have noted that they had no advance warning of an issue, your design will indicate before failing
which is a good thing, but only if your can see your square part.

Looks cool, but if you add some additional  tension (wire or solid rod) from the bottom of the chainplate to the bottom corners as far out as possible you would add much to the strength.

May you enjoy fair winds of much less than 40 Kts.
Norm

Lastly;  you did a new rudder Right?

If you did how is it working out,  have you had any issues? and does it stay centered when not under sail?