Annoying Rattle

They need more Poon in their Tangs. I agree with stiffening it.
My thinking at the moment is about bending up an inch wide strip of thin stainless steel to run from left to right, beneath the rear fender 'legs' and link it up to the fender fixing screws. With a couple of extra M5 or M6 nuts & bolts (dome head socket) into the strip through the fender above the tyre area, will hopefully fortify the structure over all.
...Might come up with something better before winter project time, but it's my initial thought.
 

sounds good for a plan a
 
well, the bike is out of Triumph warranty, and the extended warranty won't cover it, so i'm SOL on that, looks i'm buying two new sides, and i was thinking the same thing, a strip of metal run along the inside for support and to stiffen it up, i'm guessing the extender didn't help the situation, along with our amazing roads, I removed the extender and you could see where it was touching the tire. live and learn i guess, can't wait to see what those pieces are going to cost me.