Thats what I do . With a 12 inch trowel.
Where can I buy hushmat or the roadkil products
The problem with roofing cement/tar is it has a strong chemical smell that can be a problem inside of a closed vehicle, especially when the heat raises.
No appreciable smell once it dries. My wife has a nose like a bloodhound, and it never affected her.
Cutting the foil to fit, working it around in the door panels, getting it to lay flat and stick where you want it is one big PITA.