Wyo1,
Nice to see you back at it, and posting again.
Regarding the rivets used for the lift panels, here are a few thoughts. I did use 1/8" diameter for the hinges, and I apologize but I can't remember the length, but somewhere between 3/8" to 1/2". The correct term for the larger head is "large flange"rivets. You can find them by doing a search on eBay, but it may take some looking for all aluminum, which is best for avoiding rust.
You could take some of your scrap material and do some testing for length with standard flange rivets, keeping in mind you may need to flatten them a bit, especially on the center hinge for complete folding. You need enough length to get enough of the rivet crushing to hold the work. Too short and they barely crush before snapping off. Choosing aluminum composite paneling sounds like a good idea, I'll be anxious to see how it turns out. It occurs to me that you may not need the large flange rivets in your application, since you may not have the same issue of wood that crushes a bit around the rivets, especially when flattening them out a bit, as discussed above.
Shellback did his lift panels out of aluminum panels as well, so perhaps he can chime in on the rivets he used.
You could call ATC to ask specifically what size rivets they use, as well.
hope that helps a little.
Poky