I built something similar two seasons back it it works really well. The tomatoes love it - I usually have them sprouting in four or five days (most of them are up now and they were seeded last Sunday - six days ago!).

Incandescent elements work the best and they typically have a higher wattage output(about 1.8 or 1.9 Watts/foot so a 12 foot "rope" gives off about 20W of heat). The LED type has a lower output - about 0.75 W/ft - so not as much heat is produced.

I know the ropes are outdoor rated/certified so are OK to have water dripping on them (plus the ends/connectors are sealed when you "slice" them together) but, being an anal engineer type, I wanted something to dissipate the heat produced, so I bought a sheet of heating duct (2'w x 3' long) and screwed that to the top of the furring strips - I though it would keep the heat evenly spread out. seems to do the trick!