I've got a 4'x4'x6.5' tent and had the same issues with dark period temps. I purchased a oil radiator style heater and placed it out side the tent directly outside the vent slot with a large cardboard box over the heater to direct the heat into tent.. The heater has a timer so it's a set it and forget it setup. My night temps where in the low 50's F but now they are upper 60's ( 67- 70 F ). Oil Hester is definitely the safest... If u need me to I can post a pic to give you a idea.
the red glow is bad could cause hermies, be careful of some type of heaters, like the ceramic ones with a fan, theyr intended for use in bathrooms, not meant to be kept unattended, if the fan stops, they burn
i have this prob too my box gets to cold at night and dont know what kind of heater I can put inside the box, I heat exterior are but box can get as low as 44 during lights off. the heater would be touching pots or the side of box or at least very close to both so just light heat that doesnt burn ya know