Maybe a hammer, As the mower runs and turns it would tighen the bar.
Allow me to elaborate some on this hammer idea that
reynoldston has . I call it my red neck impact. Now hold on there just a minute before you make judgment too quick. This is going to be safe.
Tools you will need.
Breaker bar and socket to fit nut. The correct offset box wrench may work if a breaker bar isn't available.
Cereal box top. Yep, corn flakes will work even with the high cost of corn.
One motorcycle helmet with full face shield.
One athletic cup.
One 2x4 about 3 ft. long.
Stand mower on end.
With the box top, shim the wrench on the nut so it doesn't fall off and in a position it can be impacted with a 2x4.
Now with a Mickey Mantle swing impact the wrench end with the 2x4.
The wrench and nut will spin and the wrench may fly off but remember, you dressed appropriately. You did make sure everyone else was safely in the house didn't you.