Sound Proof Dog House

Off Highway (mine haul trucks)cab builders could help you. Many off road truck cabs are lined with a rubber backed lead product that cuts waay back on the external noises.

its going to be difficult to get rid of all noise, unless you move the engine to the back

When I had my gasser I use a blanket from Summit Racing..it stuck right on the back of the cover and the outer side was coated to make it heat proof...It did help but did not eliminate it entirely.

This is what I use on all vehicle that need it. I have the 3/8 or 1/2 one on the dog house.

http://www.dynamat.com/

I can see where 3.5" of fiberglass has a .90-.95 Noise Reduction Coefficients (NRC). Not sure how much room is under my dog house hood. Any more thoughts?

Camping World used to sell a kit that came with this material, a can of 3M spray adhesive and silver tape. You can probably buy the insulation at the link above or a hot rod shop. The 3M and silver tape are available at Home Depot.

I applied the insulation to the underside of the doghouse and used the silver tape to seal the seams. The silver tape is VERY sticky and will hold the insulation in place.

I had extra insulation when I did mine years ago and cut pieces, sprayed them with glue and reached under the edges of the engine box (what you would see if you were under the vehicle looking up) as far as I could and applied them. I also made sure I got a thick application around the gas pedal to keep the heat off of my foot.

West Marine or other marine supply store. They have it in a couple thicknesses and sizes. While they come preglued I used 3M spray adhesive
and it still holds 8 years later. Reduces engine heat into the coach also.

PS: I applied mine on the engine side not the cockpit side of the doghouse.

I would probably look at what automotive companies do. that is, insulation like I see on the underside of a car hood. JC whitney comes to mind or google car sound

or
insulation.http://www.stockinteriors.com/HeatSound.asp?Itemid=4783&gclid=CjwKEAiAkpCkBRCtstKQo5ia5nESJACsCikRPB5ZujzRPNrX1kHBEyolZBZPlKFBjyI4Ml372B4tLxoCoqLw_wcB