The purpose of the towel, is not only extra insulation, but to to take up the dead air space in the cooler.. Even if ya dont wrap it in the towel, stuff one around it to take up the cooler space, if not towels, use balled up newspaper.. You dont want the heat from the brisket used up to warm the dead air in the cooler. Originally, i used to wrap the foiled briskets in layers of newspapers and then cooler them.
I have 2 pieces of heavy cardboard cut to fit just inside my Q-cooler. When I cooler stuff I put the first piece over the item then a small towel or two rolled up at either end to create a dead air space, then the second cardboard. The get steamed up and distorted so you have to replace them every once in a while, but they seem to do a nice job of custom sizing the cooler.