Mine (50W) won't hold 20C during the frosts.
I find sand too heavy & messy. If anything, I use peat & keep it moist. At the moment, I just have 2 layers of capillary matting - moist. It was up to 17C last night. But, had I put a blanket over it, it could well have held temperature. That's OK for germination, but with growing seedlings in there, every bit of early & late light is more important than temperature. I think!