I dont know how many use or plan to use a heating pad for propagation but I was looking for one and remembered I use to make my own for my reptile cages. These things are awesome and are easily wired. You can add a thermostat to control the temps and it comes in two different sizes. I thought about making warmers for my up coming dwc challenge, and for under a humidity dome. I havent made one yet so I have no pics but anyone with any kinds of DIY skills can see how easy this stuff is to work with, and is safe as long as you wire it correctly and cover the ends properly. Here's a link to where I'll be purchasing it from when i'm ready to do it.
Buy Flex Watt Heat Tape for Less
All you need is flex watt heat tape, a extension cord, electrical tape, soldering iron, thermostat (optional but highly recommended) pretty much stuff you already have or for less than $10 (except for the thermostat).