Paul,
Though very hard to find up here in the midwest, check out some of the older bone yards in your area. They were very common on a varity of GM cars in most body styles. I have a few but they are VERY difficult to find where they are not rusted through. They use a redish colored open cell foam gasket ring on the back and the green paint is very close to Cummins Diesel green which can be bought in a spray can. I do see them on ebay from time to time. Sorry I cannot help but I go through lots of these and they don't come easy in up here.
Rick