I came up with a nice idea here lately


I used 12" wide by 2" think ducting isulation for sound deadening, and attached it with duct tape to the cab of my truck. it works pretty good, at least to the point where I dont hear any road noise excepting my exhuast, but that's a sound one never tires of. I'll put up a pic or two of it once I get my box mounted.

oh, and it only cost about $40 to insulate the whole back of the cab, that's the best part
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Only downfall with that is you are not adding weight directly to the panel. That's the main point of Dynomat. It's heavy and is sticky all over, this applies direct weigh tot he panel, lowering it's resonent frequency and therfore lowering the chance of vibration, and noise. The insulation you've described will work great for road noise, which is what sounds like you're looking for, but won't help much with resonent noise.

Ice and Water shield from Home depot is a great alternative for eliminating resonent vibraions. It's nearly identical in appearance and application as Dynamat.

 

well for christ sake most things 2" thick will provide at least some sound insulation

 

Also if it's fiberglass it can and will retain water and rot out your truck from the inside if you have a leak in the slider ect ...