Dave - good eye. The sides are 3/4 inch thick. Overall dimensions are 8 5/16 wide, 10 5/16 long, and 6 1/8 tall (plus top is proud by about 3/8).
Built so the typical size plastic temporary container for ashes (which are also in a plastic bag apparently) could slide inside. Per a call to a local company, the typical temporary container is 8.5 x 6.5 x 4.5. The interior of this box is about 8.75 x 6.75 x 5.5. I think I may buy an empty temporary container so I can be sure that will work. Watched one video where a groove had to be added to allow for the temp boxes hinge to clear the side.
The thicker of the two potential bases I currently have is 3/4 inch.