This is an older skylight that I've had in my living room for 29 years. It never leaked and doesn't now. However, there are numerous spider web-like very tiny lines that have formed around the perimeter of the outside dome of the plastic. It has an aluminum frame that "used" to let condensation form on the inside of the aluminum frame. So I bought a liquid rubber to coat the outside of the aluminum frame. It completely stopped the condensation. But a little bit of the liquid rubber touched the edge of the dome and over time those tiny lines showed up. But, as I said, it doesn't leak, nor has there ever been any kind of steaming up between the two layers of plastic.
It seems like it would be ideal for a greenhouse/garden situation. The dome itself can be unscrewed from the aluminum frame. At any event, it's free for anyone who can put it to use. I live near Ashby Ave and San Pablo Ave. in Berkeley. Please get in touch with me if you'd like it. It is, btw, quite lite, so it's easy for one person to move around.
The outside dimensions are 50 1/4 inches. The inside dimensions are 50 5/8 inches.