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Greg I use the DewNot's (got them direct for just over haldf the price of local suppliers).
Unless your skill's are exceptional trying to make robust heater strap's which fit in with the practicalities of astronomy looks like a wasted effort.
You will also want some kind of controller, a range of options (I've made my own and published details in the Projects area as have others). Getting things too hot wastes power.
DIY generally involves some kind of PWM (Pulse Width Modulation) device, Jaycar and Ocean Control's both sold kit's which can be adapted. There are are also full blown unit's on the market.
Have a browse at the forum thread's in the DIY area for some idea's one what people are doing.
Bob
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