Thanks for your kind responses, folks.
tumamoc, we should have a look for a smart solution of your space problem, if I'm for the next time in Tucson.
Otherwise you should join the TCSS! There are a lot guys who would help you too.
Sharpy, the cold frame is made by Beckmann. It's a German manufacturer. I'm not sure whether you can buy it in the USA.
This model is expandable to endless. Here
you'll find their online-shop. (Sorry it's only in German)
Only when the last tree has died, the last river has been poisoned and the last fish has been caught will we realize that we cannot eat money.
(Wisdom of the Cree Indians)Cacti encyclopedia