My basement stays at 52F constantly year round, and that's where I keep all of my grow tents and cabinets. My main production setup is a bit strange, but it may lead you to ideas of your own.
I have a 4x4x7' flower tent with a 1000W HPS lamp in it. A 2x4x5' veg tent sits beside it. I have a 6" duct running through the 400W MH veg lamp with one end open, with a piece of furnace filter on the top. The other end exits the veg tent through an upper vent port, and enters the flower tent in a bottom vent port. My grow controller turns on a fan in the veg tent which is fixed to the top of the furnace filter blowing downward into the vent pipe, to blow the warm air from veg into flower when the temp gets warm enough. This keeps flower tent higher than basement temp.
To combat the light entering from the veg tent, I cut a close-to 4x4' piece of 5/8" plywood, and elevated it in the flower tent an inch above the top of the passive intakes. There are 3" gaps between the front of the tent and the back of the tent (intakes are on the sides). My plants go on this board. There's about 1' of space between the board and bottom of the tent.
The 3" gaps allow the warm air from the lower dead zone enter the main tent, as well as the cool air from the passive intakes, but any light entering the intakes including the vent from the veg tent is deadened by the board.
My main reason for the elevated shelf was more to keep my plants off of the basement floor. I put a piece of carpet on the concrete, but the board does keep things a bit warmer, especially with the warmer air circulating down there.
However, it does wonders for keeping the light leaks at bay all the same.