Mix up enough rockite. I'll leave this up to you to figure out. Pour it slowly into the mold, until it comes flush with the top edge. you can overfill it a bit and then scrape it flush after about 10 minutes. Invert the base over it to cast in the legs (you can glue these temporarily into the base with a glue gun, or use duct tape or something. You don't want to lose them to the concrete abyss.
Once it all looks good. put it away for at least 12 hours.