Only one probably not too painful and gruesome looking method comes to mind. That's the ratched tie down method described in the book five last acts (you can download it here and really don't have read the whole book, it's just a small chapter). It's basically strangulation. But most pain from hanging comes due to the compression of the windpipe which needs a lot of weight compressing the neck. With the ratched tie down method only the carotid artery and the jugular veins are compressed and that's not too painful.
Please read the chapter, it's worth to spend the 10 minutes for it. There are some kinds of ratched tie downs and it's got to be the right one. They're cheap and accessible. He also recommends to put a bag over your head, but that won't increase suffering.
It's possible that this method has been mentioned here before, but I would definitely read the chapter that was written by an expert. If another method suiting your requirements will come to mind, I'll let you know.