I have been reading the same stuff (about DIY substrate wire/cable heating)
lately.  I haven't done anything like it myself . . . yet.  I have,
however, had communications with one person who had set up a 12v 10m system
with 30gal wire-wrap wire more than two years ago, and he says that it has
worked flawlessly to date.  I have also heard of people running the wire
through airline hose and others who have laid multiple sets of wire under
the gravel, so that in case of failure alternates are in place.

I've also heard that one needs to take great care not to kink the wires,
and to make sure to build up the 30 gal with heat shrink and then use heat
shrink to seal the connection between the built-up 30gal wires and the
larger exterior wires.

Hope this helps.


P.S.  I'm getting a 20 dollar temperature controller from Radio Shack.
It's basically just a thermometer with outputs, from what I understand, so
I'll probably need to design a circuit with relays etc., but, if it works,
it will be a sight cheaper than a Dupla or Sandpoint.  If it doesn't I'll
just have a $20 digital thermometer.  I was thinking of sealing the temp
probe in a little test tube or something.  Comments, suggestions, or
experiences with this unit will be appreciated.