I liked the round bubblewrap planter so much that I cast a rectangular one also.
I used my regular (favorite) mold, but wrapped each exterior piece with bubblewrap. The collapsible inner core of the mold is not shown in this photo.
Detail of the interior corner of the mold. The blue pieces in the bottom of the mold are 2" rigid foam wrapped in shrink-wrap.
The trough as it came out of the mold. I didn't do much to it except soften the rim with a wire brush.