The heat source is a plywood plank covered in aluminum foil, with indoor/outdoor rope lights on it. We cut down some wine corks I had lying around to keep the seed-starting trays from actually resting on the lights, just to be on the safe side. Here is a cross-section:
The heat has really made a difference! All of the eggplant seeds have germinated now! The peppers still haven't, but I may just have bad/old pepper seeds... All I know is that gardening sure keeps me humble, because it isn't as easy as it looks!
While the lack of heat caused some seeds to fail to thrive or even germinate, other seeds were fine without it. The zinnias and marigolds will be ready for their permanent homes soon: