I made this for my kids to play with indoors in the winter.
The box folds flat for easy storage when not being used.
The "Solar Bottle Bulb" provides light inside the box and hold it together.
The door is one of the box flaps at the bottom and closes from the inside with a pull string.
Add an LED flashlight to the top of the Solar Bottle Bulb when the sun goes down.

Step 1: What You Will Need.

Step 2: Make the Solar Bottle Bulb

The Solar Bottle Bulb provides light inside the Fort and holds the box together.
1. Get the center of the the 7" square piece of cardboard.
2. Measure the diameter or the water bottle.
3. Cut a hole the same size as the diameter of the bottle.
4. Remove the lable and hot glue the bottle in the hole so that about 1/3 is out the top.

Step 3: Make the Fort

1. Use Duct Tape to hold 3 of the box flaps together and leave one open for a door.
2. Punch a hole in it and tie a string through the hole so the door can be closed from the inside.
3. Cut a hole in the center of all the top flaps the same size as the water bottle.
4. Put the Solar Bottle Bulb through the hole in the top of the box to hold the fort together.

You are Done.

Sun light provides good light inside the fort but if the room is dark the next step is an extra you can add to get good light.