I made a 200 gal glass box from wood, acrylic glass, ceremic heating bulb and a therm controller. It is spacious and works much better than small ones available from stores. The box has two parts which can be separated. The bottom part is made entirely by pine wood (1X12 board with 1x4 trim). I installed four wheels for easy moving. The upper part is made by wood frame and acrylic glass and can be opened from the top. Cutting acrylic glass is tricky and must be handled carefully because its very easy to crack. I use a table saw and a new blade with smaller and sharp teeth. In the center, I installed 500W ceremic heating bulb. You can get it from pet store in the Reptile section. The therm controller (as shown in the bottom center picture) are installed outside of the box. The sensor and the thermometer must be placed inside and far wary form the heating bulb (as shown in the bottom left picture). When seedlings outgrow the box, I just open the lid and cover a plastic sheet on the top. With this method, I had 4 fully grown eggplants before the weather is warm enough to transplant. I use storage boxes as tray to hold small containers. Some insulation must be put under the tray in the space directly over the bulb; otherwise, it may get too hot on this spot.