I don't have any thoughts at this time. The insulation you provided a link for may compress enough, to work.
Another option is not replace that insulation, or do some trial runs without the insulation replaced. When I did my upgrade, prior to installing the second element, my insulation was burnt off; just behind the element. Other member's posts have also noted that when they were doing an upgrade. The back will get hotter in that area, so keep the smoker further away from combustible materials. I leave at least 12-inches between the back of the smoker and the wall of my house - for safety reason you may want to keep it 3 feet away, incase there is an internal fire. Also I've not seen any member that made this modification post anything about the back getting so hot that it created a problem.
Also if you do these trial runs, make sure that not much heat is radiating too far above and below the area behind the element.