Does anyone know how to build a barn or bunkhouse for gi joes?

A bunk house is simply a simplified house.... try looking for vintage, or western "doll house" plans.... or plans for a dog house for a small dog... they should give you the basic structural elements.... then just adapt the measurements to fit the GI Joes.

What about turning a 24'-36" wide dresser on its side, removing legs and drawers and any other hardware. You will have a good 36" across x 18" deep bunkhouse in which to line up bunk beds, hammocks and whatever else these soldiers need.

Also, you can make a barn door or use tarps to make it look like tent flaps being pulled back.

That sounds so cool you're a great grandma! How about those inexpensive unpainted crates it would involve some sawing to get in or if you want you can place it on the floor as it should be and that way no sawing. Just a tent looking top which when draped back allows access to the bunker. If you want to deconstruct one as well and use that wood for beds. Then you saw to size. Just use gorilla glue since it can look crude. Every time you see him you can add a crate to his collection. Wait til he wants mountains or goodness a desert! My guys played with those and so did my brother they are timeless aren't they. I currently have a Marine. So who knows you may have a little service man in the making. Wow this just sounds like such a fun project. You get to paint and it can be messy cause camo colors are all sorts of olive green if in grassy area and greys in snowy areas and beige in desert. I am actually envious right now and I don't like sawing and stuff but I would for someone I love. Oh have fun and let your imagination fly. Don't be to exact unless you'rea perfectionist. It should be fun and if you make the hardest part fist he can or may wish to help you, for the sake of his men😉.

Youtube has all kinds of videoes for 12" action figures or for what they call "play scale " or 1:6 scale Action figures. Barns, houses,or just individual rooms with good instructions.