old trick but a good one
tie a small diameter rope around the centre tube where the centre tube meets the rear beam (Venetian blind cord is ideal) so that you have 2 ends hanging from the centre tube...... about a foot & a half of cord hanging from the tube is a good start

then tie one end around the cross-bar in a loop on one side of where the tiller extension is fitted....
tie the other bit of cord around the cross-bar on the other side of the tiller extension.....


by adjusting the length of the cords you can limit the amount of movement the cross-bar travels

By doing this your not slamming the rudders across that far that the boat looses all its momentum and your boat speed is maintained at a higher speed.
In all reality you don't need much more than 45degrees of swing on the rudders in each direction so that's a reasonable starting point, just shorten the cord up from there as you get used to the restricted movement.
Its a simple "fix" for those that tack like they are sailing a laser