Long ago, I conjured a front rack mount from an aluminum bar across the seat struts on our Tour Easy recumbents, with a spherical washer soaking up the angular misalignment. The rack on Mary’s bike developed a serious wobble due to a missing screw, which was easy enough to replace:
From the side:
It’s a 2 inch screw sawed down to 1.5 inch, ground to shape, then run through a die to clean up the threads.
The nylon lock nut over on the left should keep the screw from working its way out of the tapped aluminum bar. On the other paw, a dab of Loctite survived nearly a decade of heavy loads and vibration.