When I eventually build a seedling house I plan to build benches with lids like the ones in this video.
https://www.youtube.com/watch?v=rZS15J2w9gc Double plastic, cheap and easy, and easy access. Probably wouldn't even need heat but I plan to have a heated water system using a water heater. Tubes run under a thin layer of sand the plants sit on. This would keep it warm and moist and cost very little (in my climate).