Build it out of insulation foam :)

And add a heater that has a thermistat...I do it on my back porch and it's working well.

Good luck.

When you build this don't forget the floor. Put insulation on the floor also. Heat will travel to the coolest spot so be sure to seal all seams and make the door so the you can seal it when it is shut. I'm in zone 9 in Florida and I get away with making walls out of 1 x 2 lumber and covering both side with plastic foil, or what ever it is called. Visquine maybe the name of it. If you have the space give yourself enough room to move things around in there. Ventilation, heating, and light will have to be kind of a what works for you. Please be careful so that you don't create an unsafe situation. Make sure all electrical circuits are fused and all flammable material is out of the way. Hope I don't sound too critical on this stuff but you need to keep all of this in the plans. Wish you good luck.

asparagusfinger