I've been trying to design my own in my head for some time. I had one that worked fair but it didn't weather the elemanets well.
So what I've come up with is I'm going to use an old camper frame as the bottom base so I can add an axle to make it mobil down the road.. From there either a 2x4 or possibly 4x4 corner frame about 5x8 feet, intended for 2 shooters. I will have a full wood floor and only a partial wood frame for now until I can get a little more money to finish it, But it will all be enclosed with shring wrap which will weather good, withstand the wind and winter, and make it comfortable. There will be a bench in the front for rifle shooting and a full pocket on the right side and rear to catch the brass. Full roof but covered only with shrink wrap. I'll have a couple old chairs in there and maybe a place to mount a powder measure and loading press, not sure on that yet, but I know down the road it will be and much fancier than that.
Now all of this is my design, but you could eliminate the metal frame and just use 2x4s and make a 4x4 house covered with what ever. Window screen, shring wrap or what you want, the point is you won't be searching for or losing brass anymore.
When I get mine done which I hope will be before the end of summer I will post a couple pictures. I'm still on another project right now.