My reason for building this was that Last season I started growing my own transplants for the first time and it went horridly, until I made a make shift chamber and it went like hot butter on toast. Previously I had a local greenhouse grow them out for me. Because I was in the middle of the farming season when I needed to build this I took several short cuts to get it done quick and extra cheap, I did a very crude test before doing it this way.
Lessons: you can get by with only a small crock pot, a fan and wood shelving to double the shelving. I used a stand up freezer unit I got for $10 bucks at our local recycling center and it can now hold 18 flats easily. I had to lose the added shelves in order to germinate the lettuce to add clamp on lights that I already had. The light had too much heat so I had to shut off the crock pot and lost humidity AND still had to crack the door. Professional units have exhaust fans to compensate for excessive heat. With the 2 make shift fridges one light and one dark I had a total of $40 bucks in them and the performance was great with a little adjustment. The main problem I had with the crock pot was it running dry.
THE NEW AND BETTER: Now that I am not in the middle of a farming season with things going awry I am shoring up a proper build. The lack of a system for controlling heat was way too much of a head ache and caused a losing battle with humidity which didn't inhibit germination for me. A few things I am adding you could think about are painting the front with chalk board paint, and adding a cut off switch for the water. LIGHTING: to one of the units I am going to cut a hole in the side to add a cheap florescent lighting fixture behind Plexiglas so not to add heat, with a fan, this is the way self contained pro units add lights. MONITORING: I am also going to add a cheap indoor/outdoor temp/humidity device...I question if a cheap greenhouse alarm is at all needed.