I replaced the foam insulation on my Northern Lights generator enclosure earlier this year using a composite foam product available at Fisheries Supply. Here's the product description from Fisheries:
"This is a foam noise barrier with an internal layer of extra dense vinyl for superior sound absorption.
It provides an effective treatment for airborne noise radiating from engines, and other machinery by absorbing sound, as well as reflecting it back into the engine compartment.
The standard composite consists of a layer of mass-loaded vinyl sandwiched between two layers of polyether fire retardant open-cell foam covered by a thin, mylar silver vapor barrier facing."
I got the kind with an adhesive backing and stuck it on the panels of the enclosure. Hard part of job was getting all the old adhesive off the panels so I had a clean surface to stick the new foam to. A lot of scraping and used a fairly heavy duty liquid adhesive remover to dissolve and loosen it up.