Ian - here are a couple I've made for my 12" and 8" SCT's and all of my refractors. I made both of these straps as well as the pulse-width modulated heater controller described here:
http://www.backyard-astro.com/equipm...dewheater.html
The number and type of resistors is determined for each application by a fantastic spreadsheet provided by the above poster - Al Sheehan. It's worth its weight in dew.
I usually use 390 ohm, 1/2 watt jobbies.
For the straps themselves, I used the resistor method because quite frankly nichrome wire sucks. It's hard to work with and connect in a circuit (won't solder) and breaks without warning. Being a series circuit, one break and Nichrome's gone. The difference with my resistor-based heather strap system to most is that I lay the resistors down in parallel across two lengths of flexible copper braid - or solder wick. Soldered quickly, the joints remain small and wet, without solder spreading too far along the braid so that flexibility is maintained. This strip assembly is then laid down on a strip of flexible plastic about 12mm wide and all of it is then encased in a tube of heat shrink tubing.
I also sometimes leave out the plastic strip and encase just the resistor assembly in heat shrink so that it remains very flexible - so that it can be used wrapped around eyepieces (with velcro) as well as being used in places such as my red-dot finder optics.
Making these are fun - and effective. I've never-ever had a dew problem. Power usage is minimal - and even my 56-resistor 12" SCT strap only draws about 0.75 (measured) amps when in use at full bore. Their positioning - inside the flange, rather than outside the tube, makes them far more efficient and I use a fair amount less power than some of the professionally made units. You want to heat the AIR at the corrector plate interface - not the corrector plate or telescope. This way, you effectively achieve the same thing - keeping that air's temperature a couple of degrees above ambient, without having to heat the hardware itself - which is just a big heatsink.