What if you wrapped each brick in black plastic & taped them off like little presents ?So I pulled my concrete bricks out of the black bags in the greenhouse. They didn't hold heat at all. So.... now I'm thinking they must be clay bricks to hold heat for a long time. Anyone know?
That, and I just do not think there is enough solar heat now to get them hot enough, and there will be less this winter.
Maybe you can try just a plain brick (no plastic) heated somehow ?
On top a wood stove or on a furnace vent ?