100% silicone should work! I got some food grade stuff on McMaster-Carr, but I'm guessing the HD/Lowes stuff would be fine as it shouldn't contact beer.
If you order from www.metalbytheinch.com, give them a call. They bent mine to desired circumference (which resulted in cheaper shipping) and were very helpful determining best thickness. I was very impressed with their services.
That is a sweet setup! I checked the "metal by the inch" website and their prices are reasonable, but a little pricey to ship. Bobby, do you recall the dimensions of your aluminum sheet? Does 19" X 55" sound about right? I'm looking at thinner gauge so I could always fold over ends if 55" is too...