Here are some extra details.
There is no fan. I hope natural draft is sufficient. The air can flow in through an opening on the front of the orchidarium. (red arrow). and out on the top.
Indeed it is difficult to see how large it is. It is not very big since the kiln a have to bake it in is not so big. It is approx. 35x50 cm
I did not sculpt any rock like features because, indeed I expect it to grow over with moss. But this wil take perhaps a year:-(

It is in my living room and right now the min. night temperature is 15 celcius. I dont't use any heating. There is some heat comming from the 2, 18 watt PL lamps. In summer the max temperature might rise to 30 celcius. But there should be a cooling effect from evaportion, I hope :-)
Right now the orchids inside are an Aspasia Lunata, a Spiranthus spec, and a masdevallia spec that is flowering continiously.
But they will grow to big. Perhaps I can grow some miniature orchids from tropical mountain regions.
I'll keep you informed on my progress. It may take a while. If you guy's have suggestions on which minature orchids I could grow I would appreciate it.