incubator with heat element on lid?

Thus far I have been making my styro box or cooler type incubators with heat tape at the bottom, a fan blowing down on the heat tape, then an egg crate shelf above that.

Does anyone make theirs with the heat tape on the top lid instead, with the fan blowing upwards? It would eliminate the need for the shelf and give more space, and just easier to make. All the electronics are on the top lid. Water bottles on the bottom with egg boxes straight on top, no need to juggle space between fan and bottles.

Also, fo you REALLY need a fan for a smallish bator?