I don't think that an aluminum disc is the best choice. If the aluminum is heavy enough to withstand the considerable heat, then it is going to weigh something. If it is thin enough that it weighs hardly anything, then it will deform or melt.
I've been down this road before. I have used steel can lids and titanium foil. The trick is that you need to punch holes. The holes are numerous and large around the perimeter of the steel or titanium disc, and there are no holes at the center. This allows very little heat at the center and diffuses the heat toward the outside.
Steel can lids are free. Titanium foil weighs less, but it costs something.