Using the heating pad with the automatic shut-off proved to be impractical.  It only remains on for 15 minutes.  There’s no way I can be running downstairs to reset it that frequently.

On the other hand, the smaller, old-fashioned heating pad works fine.  I’ve set it on a timer to run for a few hours, followed by a short cooling-off period (to prevent overheating) and repeating for most of the day.  I will move it manually from tray to tray so that everything gets its share of the warmth (and the love).

I may look into inexpensive heating pads for dedicated seed-starting use.  Realistically, though, this may not happen in time for this year’s seeds.