I had the same thing happen on my Edge 540, when it was still new. It has 2 bolts on each side, but one fell out completely (found it in the tail) and the others were loose.
Since the Yak also has nylon bolts against smooth hardwood, I suppose they work loose the same way.
I installed washers cut from plastic screen material, and also tried some thick rubber washers. Both seem to work fine... Haven't had a bolt come loose since.
I also like your wire clip mod. Wild Hare also has it in their Yak 80" manual.
ORIGINAL: Uncas
My story: After a flight last year when I was fueling up my plane I noticed the wing screw had come loose about 1 inch - maybe more and it scared the crap out of me.