If your green house is small, but pretty tight, and you have space for a bucket or tub of water you can get a setup going described here:
https://homeguides.sfgate.com/...
either get black tubs or use black plastic to cover whatever you are using. Make sure they are in the sun during the day and then near the plants that need it most at night. If your green house is not too crazy large this is not that hard to accomplish...
