I am putting in a 12" x 36" fixed RV window in my roof so when I am lying down I can look at at the stars. Not a full open or clear top but I decided it would do. I will have a curtain that can be used to close it up if and when needed to block the light. I got my window off ebay. It took a while to find one not tinted but I finally did.
Here is what it looks like so far (My build is not done)

Before I started by build I also looked at Zach's build the X-II open top (link in post above) and also at Bob Henry's build with a clear lexan roof. Here is a link to a picture of his build.
http://ww.tnttt.com/viewtopic.php?f=2&t=54695Both build gave me excellent ideas for my build
You can search and find several posts on other builds with pictures of different windows and ideas
Dan