I use a grow mat to start my Geraniums and Impatiens. Mine is 9x19 and holds 1 tray. I paid around $30 for it. I get better germination with the mat than I did without it. Last year I got 241 Impatiens plants out of 250 seeds. Not bad considering the seed house said there was an 85% germination rate.

When I feel the mat it feels barely warm but, when I feel the bottom of the starter pots it is very warm even with the tray between the mat and pot. You will get best results if you put a cardboard or styrofoam under the mat so it radiates the heat upward.

A little more.

They do sell heating cables that go right in the soil, but you'd have to start them in an open flat. I don't know how much they are. It seems to me your best option is the mat and a shallow container that only holds a couple of inches of soil. That way the heat will get to the seed. You can always transplant into bigger pots after they get their first true leaves. If you didn't know--the first leaves on a seedling are seed leaves. The true leaves are next.

Thanks for the help!