Whether you're reenacting the French Revolution or screaming "Off with her head!" a la the Queen of Hearts, this DIY guillotine is the way to go. And based on the video, it looks easier to assemble than some shelving units.
The Sokkomb guillotine is a "new low-cost product designed specifically for all those citizens who are so interested in Do-It-Yourself Justice." Yeah, it's just a mock up, but the video itself is pretty much a DIY guide without the measurements. (Does anyone have thoughts on how thick of a blade that appears to be?)