I found a better contender , silicone foam , extremelly flexible , cheap , optical clear and can be bought as a foam composition.
I read somewhere , they burn the whiskey casks inside and inject hot steam on to it and wood develops trillions of foam like structure. May be they apply heat on to the surface of plastic and than apply a chemical vapor to turn it in to a foam.
And polyurethane foams once used at boat , aircraft building. Without need a glass fiber wetting , you brush the epoxy on to this expensive foam , epoxy sucked by plastic and it sets inside.
So polyurethane comes 1 liter 3 dollars and expands in to 55 liter foam. There are transparent PU patents also.
There are many silicone and PU manufacturers and prices damn cheap.
You can consult them.