Substrate heating coils

I finally broke down and decided to try DIY substrate heating.  My
basic plan is that of Dan Resler and Uwe Behle, but I want to build
mine out of locally available materials.  I am going to build a 30W
heater for my 70 gallon tank (footprint 48" x 18").

I've decided on a 24V, 2A transformer, and 30 gauge wire-wrapping
wire, both from Radio Shack.  The only major problem is that I need
200 feet of wire to get 30 watts of power.  I'm going to do this using
4 reels of 50-foot wire, wound in parallel on a frame built of solid
steel-core, vinyl-insulated clothesline from my local hardware store.
The ends of the reels will them be soldered together to connect the
four reels in series.  The frame will be glued to a 46" x 16" piece of
plastic carpet backing and placed at the bottom of my tank.  Add
gravel, and I should be all set.

The only tools I'll need should be a drill , soldering iron,
screwdrivers, and cutting pliers, all things I already have.  
I expect the total cost of the installed heater to be under $50.

Any suggestions/comments?