Interesting. I was just fooling with the arrangement of washers two days ago to try and get better tightening. I failed.
Also interesting that the arrangement of nuts/washers was different on my new unit:

bolt head
small thin washer
plastic bracket
thick washer
thick washer
thick washer
metal bracket
open space
metal bracket
nut
plastic bracket
small washer
wing nut

I also tried rearrangements to get lock nut action, but found I could not tighten extra nut as there is no wrench that can fit inside the bracket area. I replaced the nut with a larger nut to effectively be another very thick washer - I need to get parts to do it right as it needs an manual adjustment after every bump in the road.

Al