Temp. controlled DIY heat cables

I am working on building a set of DIY cables for my 55 and was looking for a 
way to improve the recommended low wattage design that is so common.  I 
figured that the ideal is to have the cables provide all the heating in the 
tank, thus maximizing the convective flow and warmth of the substrate.  My 
plan is to take a 300 Watt Otto compu-therm heater and hook the supply for 
the transformer up to the control box instead of the otto heater.  Voila, 
instant probe and controller.  The cost of it is $30.00, which is less than 
I would have paid for another heater to make up the difference in the 
required wattage and a timer to control the intervals on the cables (if it 
would have ended up being necessary).  

Has anyone tried this?  Is there any reason why this wouldn't work?

It seems like I am not the only one looking for suitable cables to use.  I 
was looking at the cables designed to melt ice off of the eves of houses and 
out of the gutters.  They are waterproof obviously, but I didn't know much 
else about them and didn't want to spend $26.00 to learn that they wouldn't 
be usable (my insulation sucks, so the heat from my house melts my snow and 
ice- no use for them their.)

Does anyone have info on these that could benefit the cause?

jsprag at srv_net