Line a shallow pan, such as a loaf pan, with parchment paper and set aside.
In a liquid measuring cup, combine the coconut oil and canola oil, stir gently, and set aside.
In a blender or food processor, combine the rice milk, coconut milk, agave nectar, sunflower lecithin, salt, and xanthan gum and blend for 1 minute. Very slowly add half of the oil mixture, followed by the lemon juice, and then the remaining oil mixture. Blend for 1 more minute.
Pour the mixture into the prepared pan and refrigerate until solid, about 3½ hours. Chop the butter into 1-inch cubes and store in an airtight container for up to seven days.