Try cutting out an old polyurethane foam cushion and then custom mixing the
soil. Lots of leaves and manure for instance mixed with the soil. You can
leave in a bottom or not, as you choose. If there is no bottom, you can use
thick layers of newspaper, cardboard to kill what is underneath, as I expect
you do not want that growing through. These biodegrade with time anyway, but
you should mulch them on the outside. The raw material is best found in inner
city neighborhoods, or around big universities, wherever people are careless
about their trash. My best cushion was contured thanks to a couch fire. It
rivals in looks the best sandstone trough. When you kick it, it gives unlike
stone or hypertuffa.

If you want you polyurethane foam custom made, expect to pay.