There is this option.
http://www.dewbuster.com/
For the price you get temperature based control on two outputs. This is not available on the DigiFire7 but is available on the Digifire 10 and the higher end Kendrick unit.
Combined with these Heaters Dew-Not.
http://yhst-1469288697390.stores.yah...ewheaters.html
I believe Peter from SDM can get both of the above for you.
Alternatively AndrewJ posted some good plans for a temperature controlled options in one of the threads here.
Regards
Fahim