Dynamat is better but expensive. I located a similar material on the web and didn't go mad sticking it down so I can always pull it up if I ever need to do other repairs etc.
It's a self adhesive sound deadening/anti drum pad. I haven't tried to get any for years but bodyshop suppliers used to keep it and may still. On floorpans with many shapes, it has to be heated almost molten to sink properly into shape! It also traps water which is not what you need! May be better to choose a modern alternative. I only really used them when replacing doorskins, to stop the panel drumming.
Steve
'73 2 door Toledo with Vauxhall Carlton 2.0 8v engine (The Carledo)
'78 Sprint Auto with Vauxhall Omega 2.2 16v engine (The Dolomega)
'72 Triumph 1500FWD in Slate Grey, Now with RWD and Carledo powertrain!
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