I saw a similar idea last summer using plastic sheeting. My problem with that is you'd die in there in 10 minutes in the desert in the summer (oven anyone?!) so I've been wanting to try it with something a little more breathable.
We don't have a small box fan just a tall skinny one. Instead of waiting for the perfect materials I decided to make it work with what we have. After all you all are going to be working with whatever you have at your house which isn't all the same!
I used a duvet cover since it's already secured on 3 sides but if you don't have one just secure 2 flat sheets together (you can duct tape or sew the ends together) I used duct tape to secure the open end around a fan and left an opening for the boys. You'll need to have a way to secure the opening once they're in. Binder clips, clamps, or clothespins are great for this - especially if they can do it from the inside!
That's it! A cool, breathable fan fort! The boys loved it right away.
Then this happened. And they loved it even more.
There is a dog and a kid in there somewhere.
Next week is Spring Break - wish me luck! ;)