Bill,
I took a hard look at this but couldn't quite justify it. You can't really start trees in it in the winter without adding artificial light and of course heat. So, that limits use to a short period in the spring for my purposes when acclimating plants. It would me to get seedlings out of the basement about a month earlier.
I considered a number of things before I abandon the idea. First, if I plan to use it in the summer, I would need a way to control temperature. If I use a manual method with screening and venting, I would need to be around almost daily. Would I want a watering/misting system? One would also need fans to stress trunks where the wind on my deck is free.
I think Wayne had a thread on one he was building. He doesn't post here as much as Deerhunter forum. I don't think he likes the harshness of some folks here. If he doesn't answer a PM here, PM him over on that forum.
I only dug into to it enough to table the idea for now. It is still something I'm considering for after I retire and move to a new house, but I'm not sure I'll still be growing seedlings then.
Thanks,
Jack