The only thing i can come up with is to start with a long mitre on the edges, use a jig to cut a 45 degree dado on each corner, then inlay the edges/feet?
My concerns are:
- I can't use splines on the mitres, they'll be visible.
- I'll be cutting most of the mitre joint away, leaving very little glue surface.
- I'd have to glue in the feet/edges cross-grain, so the glue will probably fail with wood movement.
The upside is that this is an urn (i guess that's not an upside for everyone involved) so I'll be gluing the lid on, which should provide some extra stability.
That's what I'd do. Maybe try for thicker sides so you have a bit more meat for the mitres. Or you could just flatten the corners and butt the edges with glue and tiny dowels.