flounder33
The best design I have seen was in a winter shooting shack in a club I belonged to before I moved south.
Its real easy to put together. you make a loop hole like the early settlers did to fight Indians, then a door that slides over it to seal it when you are not shooting. The one in the club house slides up and down and there is a weight and pulley set up so when you want to shoot you take the weight off the shelf and let it hang it opens the door when you have fired you put the weight back on the shelf and the doors own weight closes it.