This is a quite cool project with a peltier module. By using a DPDT (a switch that can swap polarity), the peltier module can be adjusted so it heats up on one side but if polarity is reversed that side will not decrease in temperature.
See my video here: https://youtu.be/s2NHbMaGHjg
Top solid layers: 3
Bottom solid layers: 3
Sliced with Simplify3D, and estimated printing time is 2-3 hours.