For what it is worth, human ashes are usually returned in a sealed very heavy plastic bag, loose enough that it can be reshaped to fit whatever container it will be stored in. The crematory loves to sell overpriced urns, but lacking that, usually puts the plastic bag in a plastic box (sometimes made to look like cheap wood).
Two points (if the human analogy is true)
1. You don't have to worry about the exact shape of the final urn, just the total volume. From the dimensions you gave, there was room in the box for a pet over 100 pounds live weight.
2. You don't have to worry about the ashes sifting out the bottom or otherwise leaking, unless the owner opens the bag and wants to play with the ashes (such as mixing ashes from two pets, like people sometimes mix husband and wife ashes).
When you deliver the urn, I would find it useful to learn the similarities or differences of the pet crematory practices.