Browndrake, there was a post a while back about a guy (I believe from Texas) who was going to do much the same as you would like by excavating a pit and covering with a clear geodesic gome. If you have a friend with a backhoe you could ask him to dig a layered pit and cover with the dome. I've seen geo dome frames in ebay fairly inexpensive that are meant to be covered in shrink wrap(used here a lot for boats in winter. I don't know enopugh about you climate to know how practical this would be but this could be a fairly inexpensive and do-able option. If it were me I'd provide extra heat and humididty with a built in hot tub..........now my mind is starting to wander.
