Nest box heater to keep eggs from freezing

Has anyone seen a way to make a heater for a nest box?  When it's 0F outside I need to get eggs every couple of hours or they freeze.  If I'm gone and the chicken sitter is coming by once or twice a day, the nests become a horrible mess of broken, frozen eggs.  

I found one nifty homemade system with a seedling heat mat (Backyard Chickens)
I also found a heated pad that could work but it would get expensive for 5 boxes (Ebay)

I wandered around Menards today looking at options.  Some things that came to mind and the possible issues are:

Incandescent Christmas lights in the plastic tube.  Route them around the nest box bottoms to provide a bit of heat.  Could be turned on/off with light timer.  No thermostat but that's probably fine.  Glowing bedding might be a problem?  Here's a blue incandescent one I found, 3W per foot (Menards)

Pipe heat tape.  It's made to keep things just above freezing.  Problem is that I think they rely on a built in thermostat.  So if that thermostat isn't touching an egg, it would just stay on and possibly get too hot?

Gutter deicing cable.  These have a thermostat and I believe a water sensor.  Tricking them to come on may be a challenge.  But I already own some from a previous house that had ice dam issues.

Under floor electric heat mat (often embedded in the mortar under tile in bathrooms).  These are a good shape to cover the bottom of the nest boxes (the boxes would be built to handle one long mat).  But they're pricey and they'd either be on or off OR I'd have to figure out a thermostat.  If they're just "on" all day I'm guessing they'd get too warm.

Seed starting mats.  These would be nearly perfect.  I have one already.  But it's too short so I'd need to get another.  Mine is controlled with a thermostat that has a probe in one of the seedling pots.  I could plug it straight into a light timer and skip the thermostat (as in the DIY guide).

By typing this all out, I think the answer is to just get another seedling mat.  Or do the rope light on a light timer.

Anyone have any other ideas or thoughts?  The device would only have to be on from daybreak until 4PM.  It only needs to bump the egg temps by 20-30 degrees F.

Just found another option...  Reptile Heat Mat

It's only 7 watts so it seems to be about the right amount of power.  And if it can go under a lizard, it can go under a dinosaur.

Looks like a rope light would be $25, a big seedling heat mat would be $50 and 5 lizard mats would be $60...