Dave and Woofie said the most important things. A towel or some plywood are probably 90% as good as drywall.
One thing: as soon as any seedlings emerge, you really DO wnat to remove the humidity dome. Those domes assure 100% humidity and still air, which is almost a guarantee of damping off.
Sometimes, if half the tray has emerged but the other half hasn't, I'll remove the humidity dome but drape some Saran Wrap over the half of the tray that has not emerged, to keep those seeds moist. I use 2-3" lengths of mini-blind slats cut thinly into halves or thirds lengthwise to support the film up off the soil-less mix (just in case touching the soil is a Bad Thing).
Usually when I start seeds in plug trays, I cut those trays into 4-5 "slices" and plant all one seed type in each slice. That way, I can re-arrange the slow germinators to be together under a dome or plastic film.
