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I've given workshops at BDSM runs on safe crucifixion using ropes. First, get some soft but thick rope. Nylon without a hard core is ideal. You'll want to make four or five loose loops around the patibulum (crossbeam) that the crucified person can easily slip his/her wrists through. This will support the person's weight. You'll need to put nails or eye screws into the back of the patibulum to keep these heavy ropes from coming back toward the stipes (vertical beam). Then you tie a much smaller rope or heavy chord around the condemned's wrists. This is tight enough only that he or she can't slip his/her hands through. NOT tight enough to cut off circulation. The cords are also secured to the back of the cross a little further out than the heavy ropes.
You need to have attached a piece of wood, at least a foot long and sturdy, horizontally to the stipes, near the ground, just high enough that the condemned's feet can't slip through and touch the ground. Using another length of the heavy rope, wrap several loops around the stipes, hanging off the horizontal piece or wood you've attached to the stipes. The condemned's feet must not be able to get through the loops. Place an eye-screw or nail at the bottom of the stipes and tie another length of cord around the ankles assuring they're only tight enough so the condemned can't pull his/her legs free, but again, not tight enough to cut off circulation.
I've stayed crucified this way for about 3 hours. Within 15 or 20 minutes, I was pleading to come down--it hurts that much--but I had negotiated an ignore any pleading agreement, and having been crucified numerous times, knew what I was letting myself in for.
I find a regular double knot tends to slip, very undesirable in a crucifixion, but putting an extra loop in the both sides of the knot usually takes care of that.
Bases are simple to make, just an H shape. Use heavy lumber. It's better to overbuild than to have your cross capsize with a living person on board.
SAFE, SANE, CONSENSUAL!