Or build a germination area in the warmest, sunniest part of your greenhouse. Seeds need a consistent soil temperature to sprout. Most seeds begin to grow when the soil is around 65 to 70 degrees. Some like it hotter, like peppers. Some like it cooler, like cabbage and broccoli.
Consistency is the key.
A 10 degree fluctuation can ruin the stand. Start with clean containers and sterilized soil. That will reduce seedling diseases later on. We use rectangular, shallow trays that have been rinsed out with bleach. We gave up on sterilizing our own soil.