This is the way I intend to go if circumstances arose for it. A guillotine is not hard to build. The frame is literally just four 4x4s. I think the blade is supposed to be loose in the grooves so there's no friction? Or am I wrong? Not entirely sure what to use for the blade. A butcher's knife, a machete or a sharpened lawnmower blade might work. Would obviously test a few times before using, on a piece of 4x4 most likely. The guillotine doesn't even have to be that high, either. You can make a short one with weights and some sort of tensioner, like a couple bowstrings tied together from a crossbow. Anyone who has ever built anything can make one of these easily.
I read about some guy that did this - set a jigsaw to go off on a timer switch to cut the cord. Set it for like, 2 hours, have a blanket and a pillow that you can sleep on, have your head locked in place so you don't miss and literally just fall asleep and never wake up. I would also drink some alcohol before hand and probably some sort of drug that puts you "out of it" so that you don't really know what's going on. What drug though? Tylenol 3s? I know nothing about drugs. I assume your head will become conscience for a split second after being severed so the drugs should solve that "omfg" moment.
If I had the time and resources, why not? I would set it to automatically drop while I'm asleep.