I considered making my own, but decided it easier to buy a Warm-n-Safe liner and Heat-Troller. I am very happy with it. The quality is first-rate and the thing works extremely well.
Now that I have one that I know works, I plan to tinker with making my own. I usually wear the FieldSheer Highlander suit (Roadcrafter knock-off) for winter commuting. I think I might wire up the liner in it using some of the techniques I have read about here.