Hi again Jake,
I contacted my Upholsterer this afternoon (he is back from holidays today) and the product he used is called dBKill. It's an Australian licensed product that is similar to, if not the same as, Dynamat.
It comes as self-adhering squares that are cut to size and heat bonded onto the surfaced to be insulated. He uses a low-heat gun and a small roller to closely apply it to the floor, firewall, rear door/body cavities before covering with carpet where appropriate. It is fire retardant and has a good reputation as a sound absorber. In Australia it sells for about $140 for 10 490mm x 490mm sheets (~ 2.4 sqm in total)
It works as a sound insulator by adding weight and stiffness to the metal surfaces to which it is applied. This then damps any vibrations.
It seems to be a good product and there must be a European equivalent somewhere that is available to you in Finland.
Chris