I purchased a heated seat pad from Walmart for $20 or so and removed the heating element. Took the cover off my seat (tip: you don't need to remove the whole cover, just enough so you can reach the area you want. Makes for much easier re-assembly when most of the seat cover is still in place.) and put the element where I thought it should go. I think I used duct tape to hold it in place.
Ran some wires and installed a simple on/off switch.
The seat does take a little while before I can feel the warmth, but then it stays nice. It doesn't transform my cold weather riding like having a heated vest, but it's a little extra that's nice.