Keeping a few seedlings warm in winter

[Tipsy]

Hi,new to the forum,but been growing orchids for about 10 years in Australia

Got about 50 Aussie Dendrobium hybrid seedling and am a bit worried they will get too cold where i live as it can get down below freezing
Is there a simple method say putting them in a styro box and maybe a heat mat under or is a grow light better to keep them warm and growing thru winter and would I need fans to keep the air moving any suggestions would be appreciated thanks,there is probably some info on these forums but I could not find anything about seedling warmth.
Thank you.