A neighbor of mine remodeled his house and I noticed a couple of used skylights near his dumpster. He was happy to let me have them. I laid some cinder blocks in a rectangle, topped them off with bricks and filled the bed with soil. Next, I laid a skylight on top of the cinder blocks, unhinged, so I can simply lift it off when needed.
I didn’t use mortar to cement the bricks together with this cold-frame design, so this way I can remove a few bricks for ventilation, or add a second row of bricks as the seedlings grow and the frame needs to be heightened. After I transplant my seedlings into the garden, I use the soil and box as a planter for wildflowers.