Some members have asked for a parts list for the outfitted plant cart, so here it is.
1. A plant cart. Pre-built types like the one used for the greenhouse run from $500 and up. If making it yourself you'll need PVC pipe or some sort of metal framework, a good way to support a fluorescent light fixture or two per shelf, and fluorescent lights and fixtures (full spectrum or daylight lamps).
2. Solar Pool cover. 12x20 will be plenty. This is a great insulator for pools and greenhouses. Assorted threads at Gardenweb will tell the story. If a large pool cover were to be stretched over the entire greenhouse of the Horticulture Society it would probably keep it 20 degrees warmer than outside, virtually all the time.
3. Small Ceramic Space heater. A radiant heater is better because it does not have a fan that can wear out, and the heat is radiated instead of blown. This heater is affordable, keeps the temperature set at 5-degree increments from 60 to 80 F, and is small enough to fit on the bottom of the plant cart.
Heaters usually draw 750 watts at low power and 1500 watts at high power. Since 1500 watts is the capacity of most power outlets, make sure that the heater has its own plug unless you plan to keep it on low.
4. 100 Plastic Ties.
5. 15 feet of Velcro.
6. An extension cord with 12 feet or more of cord. Get one that has space for transformers (for the larger plugs of heat mats).
7. Two clip-on fans to keep air moving inside and distribute the heat to higher portions of the cart. These run between 10 and 20 dollars.
If the heater is not plugged directly into an outlet, use a heavy-duty, outdoor 12 or 14-gauge extension cord (15 feet or so). These can carry 1800 watts versus 1625 for regular 16-gauge cords (which can warm up uncomfortably when the heater is on full power).
Measure the pool cover according to the widths of the metal framework, cut out pieces that are a few inches wider on both sides, and attach them, taking care to overlap pieces to retain heat better.
Leave 6-8 inches of slack on the side that is to be the "door". After attaching it (no ties on top), apply the velcro all the way down. Let it stick for a day - don't use it during that time.
Finally, secure the top overlapping portions with plastic ties.