The outer surface of Dynamat is a metal foil so I believa that it should be waterproof.

Not sure if this is the same stuff, https://www.ebay.co.uk/itm/393779132303 and seems inexpensive.

I used a similar but "quilted" product intended to stop resonance (a slightly different phenomenon), which also comes in sheets and as supplied there is sufficient for lining the boot of small convertibles, so should fit the Mog rear deck.

. Be careful it can't drop on to the battery terminals, if you have a rear mounted one.

Good point that, some of these modern aerosol adhesives are easy to use but seem to loose their bond over the years

Grumpy old man mode on ..... glue hasn't been the same since they changed it to stop the little buggers sniffing it, doesn't damage them now, but also doesn't stick two things together properly anymore!! ...... Grumpy old man mode off 🙂

K

That lovely smell of Cellulose paint from rattle cans has gone too, I sprayed a panel in the shed a couple of weeks ago with some modern version and two weeks later it still smells like a dead cat in there, goodness knows what the propellant gas is now

Butane or propane.