In looking for a good product to insulate the floor I read on a forum to use Frost King duct insulation. The pros and cons of this product made me try it. Since the car had nothing under the carpet before (except what was left from 1963) I should notice a more quiet ride. But being a convertible who knows.
I used 2 packages (15' rolls) of Frost King, one package of their pipe wrap tape and half a roll of 3M metal repair tape. All together my cost was under $60 in materials.
I think it came out great!