This is a very simple door jamb for a litter box container.  We got this small cabinet thing, and I put it on casters and cut out an access door in the front for the cat to get in.  The wood was just ¼" ply or so and had jagged edges since I just used a jig saw.  I made these so it would cover the edges and be nice and smooth for her to get in and out.  It's also small enough that the dogs can't stick their head in and eat all the tasty treats inside.

Made from PLA.  Message me if you want the fusion 360 file.