As mentioned earlier. EVA 75 is what you want my friends
. Buy it from Clark Rubber. Comes in a good sized roll.
My 12" and 4" utilize them nicely. See the pics. Easy to work with. LIGHT and flat black. Just add Velcro with a self adhesive back. You can also use it to make dew shields and light shields for finders and telrads.
Also the only dew that builds up on them is this ultra thin film of moisture. Unlike an OTA which can feel like it was left in the rain.