From my ToyotaNation thread...
Under belly pan... - Page 3 - Toyota Nation Forum : Toyota Car and Truck Forums
Had to pull my belly pan today when I noticed my side skirts were drooping more than it should. The main support beams had cracked at the fastener point even though I had some basic round washers on there. I guess it's from all the bumps in the road. At least it didn't fall off while I was on my road trip...
Looks like need to go with some rectangular washers or with some thicker "C" channel...maybe in aluminum. Might be kinda pricey though. I'll have to check with the local metal scrap yard and see what they might have.