From my research, each 1 pound equates to 1 cubic inch.  I have decided to construct the urns I'm building to accommodate the typical plastic temporary urn.  I picked one up today.  Thankfully it fits easily within the urn I already completed.  The person at the crematorium suggested making some extra allowance (about 1/8 inch) as some of these temporary boxes have a different type of clasp that is proud of the side.