A method that I have been playing with recently, is to cut the shape out of a balsa sheet (type of wood not important). I then jam the pattern into the hole half way. The cutout does not need to be a perfect fit, just tight in two or three places.
This then forms the bottom of the mold box. When the POP is poured, with a little agitation, the bubbles rise away from the pattern, leaving a perfect mold, no bubble craters and a flat surface. There is a little flash where the pattern did not fit the balsa perfectly, but once the mold has been fully dried, this sands off very easily, leaving good edges to the mold.
Is anyone else doing similar? If not, I think it is worth an experiment. Another advantage is that you can control precicely, how much of the pattern is molded.