I used some surplus 1/2" thick Neoprene rubber mat. Used some 3M spray on rubber contact type glue. Worked great. Someone on the Forum here mentioned that they found a Yoga mat at a yard sale. I wouldn't go much thicker that 1/2" due to clearance issues.
I believe the main purpose of the insulation is for sound dampening and secondly is for heat control. Without the insulation the motor will be really loud. The problem with most sound dampening foam is it is too thick usually 2" and up. Although, there are a few kind with less than 1" thick. And finally, you want to make sure the foam is fuel resistant as there will be times when you tilt you motor and dump some fuels from the carbs.
Yeah, I use the yoga mat. It works adequately. It is thin enough to not cause interference yet provides some sound absorption. I use contact cement and cut the pieces to fit. Remember: no insulation is going to damp a lot of sound; there are two big air intake holes through which sound escapes--one in the rear and one under the front of the lower cowl.
HOWEVER: If you have the old style "refrigerator" Chrysler or white Force the coves are either fiberglass or plastic in two pieces riveted together. They can vibrate because the rivets can come loose and they are only spaced about 8 inches apart. I remove the rivets and take the cover apart. I glue the halves with epoxy (fiberglass resin will not adequately stick to the plastic covers. I replace the rivets with 1/4 inch bolts and locknuts and replace the mounting hardware and trim. The cover is now one piece and will vibrate less.
Before attaching the trim I fill in all non-necessary holes, remove decals, sand prime and paint the cover. Then I apply the full size mermaid decals and letters from Chrysler automobiles.
Go online and google mermaids. You will find thousands of photos. That particular mermaid is a piece of fine art by a Polish painter--forget the name but the painting IS hanging in the Wausau museum. It is whimsically entitled "Last Baltic Mermaid Catching Bird Flu."
He made a couple of very different paintings including one of a mermaid with her boobies hanging over the edge of a pool , looking at a graphic photo of her private parts. She is saying " Darling, I have so many times told you not to photograph me" Not suitable for public display on an engine or in the forum.
Anyway, download the photo you want and bring it on disc to an auto decal maker. Download the file as large as you can make it to avoid pixelation or lines when it is blown up onto the decal. It will cost about 75 bucks for two mirror image decals. I also found a mermaid online called "Hippocampus" It is a fantasy painted by a very famous fantasy artist called Boris--something-I forget. I bought a print and had that blown up into a decal.
This photo is a cover in progress Pay no attention to the date. I never update my camera.