There are five cross-bars that attach to the middle bar, made of PVC. The top and bottom of the bed (lengthwise) are open, to let air and light in. The fabric is duck canvas, but any fabric without too much stretch should work. I don't know, maybe stretchy fabric would be awesome (or more awesome). We wanted it to be pretty heavy-duty though, in case someone decided to try and lean on it.
Another view of the top of the tent. The dimensions were based on how high our ceiling was above the bunkbed. And we gave it the shape of the top half of an octagon (is that an A-frame?) because we have a fan in the room that would have hit it otherwise.
Yet another view.
We attached the fabric to the PVC pipe with heavy-duty snaps. We also reinforced the fabric where we attached the snaps with pellon (fusible interfacing, or for those of you, who like me had no idea, it's thick stuff you can iron on to your fabric to make it more stiff and sturdy.) Mike sewed on ties at eight points--one in each corner and two more on each side.
Here's a picture of the snaps unsnapped. In the top right corner is a little window that I cut out and sewed, after we took the very top picture.
The door way was the hardest, Mike said. This is where the ladder comes up. I don't even know how to explain it (I'm sorry). He sewed the fabric so it made a pocket for the pole. The gauzy fabric in the top picture is a "door" that I sewed on later because my daughter wanted to be able to close the door. We chose the see-through stuff because we didn't want to block out all the light.
I sewed some little pockets on the inside so she could store her things. She also liked to put things in the fold of the fabric on the sides. I say "liked", because since little Norah was born, we've had to switch things up a bit and the bunkbed tent resides in pieces in our garage. :(
Hopefully this will give you some more ideas on how to build your own. Good luck! Feel free to leave a comment, with your email address, if you have any questions and I will respond as best I can. Also, if you make one I'd love to see it, so send me pictures!