Hi randy,
I have two 4x8 tables set up with 40 feet of heat cable in each. i put a sheet of 1 1/2 in rigid foam down then framed it with 2x4's and buried the cable in decomposed granite and then dry laid some old porcelain tiles on top of it. then I put hoops and plastic over that. Looks like a covered wagon. Works wonders. This will be the third year of using them. It would work good inside your green house and if you orient it right you can grow under it, or in any case store trays and stuff under it.

The cables were $40 each and I had leftover tiles and you don't really have to have them.

there's several manufacturers. which one do you have?

Sorry Randy, I am not going to remember that. I bought them online from one of the extreme pumpkin growing stores which I can't find now. . They were not anything special as far as heat cables go. They have a built on 72F thermostat. In my mild winters and insulated raised tables that probably works and is useful but you may not need it.

Hi, was reading your thread,could the place be Holland's Land O Giants, it's a pumpkin growing place and has the heating cables.I buy my seaweed extract there.

Honestly, heating cable is so outdated. I use what the exotic pet reptile industry uses now. Heat tape. MUCH better IMO:

http://beanfarm.com/product_info.php?cPath=1204_1205&products_id=5569

i changed direction a little with this project. i decided to ditch the flats and just use the medium in the raised bed itself to sprout the ginger.
the raised bed will be like a 44'x30" flat. i'm going with the redi-heat hd cables. it looks like berry hill has it. i'm doing this to sprout 120lbs of ginger.