if you dont want to do it the expensive way they go for roof flashing, thinner than dynamat but also a fraction of the cost. with it being thinner it means lighter but you need more layers of it to equate to one layer of dynamat.
two layers on the outer skin and 1 layer on the inner skin so 'seal' the door as best as possible but if you go down this route, then please please make a mdf baffle for the driver to mount on. make sure you bolt it to the door itself. do it once do it right.
cov comparing a car door to his PA speaker with no back is bollocks! the door itself with the door card in place will act like a speaker cabinet. just make sure if you go down this route to at least seal the inner skin to help with audio reproduction. its one thing i have left to do on mine but when i done it on my old mera. fuck me it made a difference, tigther, punchier mid bass and also means you can drive the speakers a wee bit harder as they will not bottom out as easy