www.raamaudio.com You can get the raamat60 for 79$ for 62.5sq ft. Thats the cheapest I know of. Just email him with Retrodrive as the subject. It'd be pretty hard to make it by yourself. You'd need the asphalt, the glue and the aluminum backing. It took three rolls (240$) to do my entire car including the doors, trunk, floor, roof, everyting.
Roofing peel and stick. Carpet pads don't do as well, and can't get wet.
Besides, you can buy a craplaod of peel and stick for $20. for $50 you should be able to take out your interior of the car and go all the way around it. Underside if you want to clean it and go crazy¡!¡
I use the gutter patch stuff from Home Depot. It will work fine unless it's in a place that will get really hot (like the trunk lid), or it might melt. I've completely lined my front doors with it and had no problems. You just can't expect to remove it without making a mess.
Here's my $0.02..... I got Raammat off ebay and put it in. That stuff didn't stick worth crap to my trunk lid, even after I cleaned it. All of it except a few small pieces fell off due to heat, some more just fell yesterday, actually. However, the triple layers on my quarter panels are still holding. If you can keep it adhered to the trunk lid, go raammat. I'm going to reapply some in a few weeks and hope it sticks this time....
go to walmart and for $3, buy some Spray-On Bedliner. All dynamat does is add mass. this spray-on bedliner adds more mass than the dynamat, plus you can get into the nooks and crannies. it has the same effect, except better. it adds mass. thats all dynamat does.
I don't think the density of the spray-on bedliner is as high, but I'd say it's worth a try. It's definitely cheaper. You can use undercoating from a parts store, but it will smell pretty nasty for a while. I used it on the inside of doors of my truck when I painted it, and I think the results were alot better with the gutter patch on my car. I suppose it's personal preference, and if you don't like the bedliner you're only out a few bucks.