OK, looks like the next step is to cut the base with a sterilized sharp blade and let it dry and callous over. First time I've worked with cacti; what's the success rate of something like this without the rooting powder? I've only ever worked with aloe and small indoor succulents before, never cacti with barbs.
https://succulentplantcare.com/how-to-easily-root-and-propagate-a-cactus/ 