This year I broke down and got a heat mat from amazon, vivosun heat mat
and sprung for the vivosun temp controller
The temp controller has a little probe that is hard to keep in the soil of my 72 seed tray, when I was trying to harden off the seedlings and taking them outside yet keep them warm to help along the stragglers that weren't germinating. In hindsight, I planted too many different things in one large tray and should have used smaller trays. Things I germinated: Lavender, chamomile, malabar spinach, chile pepper, cucumber comfrey, tomato, kale, rosemary, hollyhock. Those were all in one tray. Then I had two pie tins each onion seedlings and I successfully germinated some honey mesquite from a large specimen in a park from some old pods that were on the ground.
I like having the controller for consistency and record-keeping, to figure out what works best for different plants. If I were germinating on a larger scale I'd probably go the route of soil-warming cables, which I think would work best in a greenhouse situation.
Do you have thermal mass in your greenhouse?