Photo 1: The space between the paper mache shell and the trashcan was filled with expanding urethane foam. This is pour-in-place two-part rigid urethane foam. It comes in two parts that are mixed in equal amounts by volume to expand to 30 times the original size and sets rigid (final density is ~2lb/ft^3). The foam was poured in several batches until it was almost up to the lip of the trashcan.
Photo 2: A shelf to help support the Liquid Tray was formed by finishing the fill of the foam up to the top of the lip of the trashcan. To insure a flat surface as the foam fills the void, a 'capping form' of wax paper backed by cardboard was taped into place between the trashcan and the paper mache in order to contain the foam as it expanded. After each pour of foam around the perimeter of the trashcan the form was relocated for the next pour.
The top edge (lip) of the cauldron was finished and strengthened by taking piece of large diameter vinyl tubing and slitting it down the length so it could fit it over the edge of the rough paper mache lip. The tube was shot full of 'Great Stuff' foam sealant and slipped over the edge of the paper mache and allowed to bond and harden.
After the flat surface for the Liquid Tray was complete it was time to fill the space under the overhang of the cauldron lip. This was done by building forms as before with wax paper and cardboard that were tapped to the lip of the cauldron and supported from expanding inward by pushing nails into the shelf foam.
To seal the urethane foam from liquid spills it was coated with polyester resin which was mixed with catalyst per the manufacturer's instructions and applied to the exposed foam of the shelf and also over the outside of the paper mache. In the future I anticipate fiberglassing the exterior to add strength and to resist damage from handling.