Carpet underlay ?

I've used some window sealing tape that is if I remember correctly it is about 6" wide and comes in a roll. I can't remember also how long the roll is but it has foil on one side and is sticky on the other. Not as thick as Dynamat but if you double it is the same thickness. I was told this by a guy that works at a car audio shop here in town. He told me to go to Lowes or Home Depot to get it and that he did his whole car even the head liner area for under $50. I got two rolls of it for less than $7 a roll and did the doors of my 03 Dodge Ram and it made the doors more solid and made the 6 x 9 speakers sound a lot better. I also bought some and put it on the back of the truck behind the seats to help deaden it for the subwoofers. The stuff works because I hit it with my hand before and after it was a very noticeable difference. I plan on one day doing the floor and doors of my 67 with it and maybe add a layer of the foil bubble wrap type stuff on top of that. I used just the foil bubble wrap type stuff in a 68 Charger that I once had and it helped a lot. I put it on the doors also under the panels. I spray the foil with 3M adhesive and floor boards/door metal and let it tack up some and then stuck it into place. But if money in no option I would get the Dynamat.