I needed to make a cold frame to sprout tomato plants while it is still winter here in the great northwest. I started by making a 2×4 frame with lap joints at the corners. On top of this frame, I attached a 3/4 inch piece of plywood to make the floor of the cold frame. I then made four sides to form a box with an angled shed roof. The roof is a rail and style frame with Plexiglas panels to let sunlight in. The roof is hinged at the lower side to allow access and heat to escape. I installed a heat actuated opener to open the roof panel when it gets too hot inside the cold frame. I also mounted casters on the bottom to allow me to move the cold frame inside and outside as needed.