At my wife's behest, I cut an access hole into the door which leads down to our basement. By this portal the cats will now access downstairs when we shut that door, which we are wont to do to clear the tight space into which it opens.
This cat passage enables us to move the second catbox from the first floor down to the basement
I had the idea that the opening should be lined with bristles such that the cats would be brushed when they walked through. Obviously I didn't have exactly the right materials to do that, but I did line the hole with the grippy half of a roll of velcro (ahem, excuse me, "hook-and-loop fastener"). And indeed, when the cats walk through, their back rubs on it and it does brush off a bit of fur -- nice.
The cat pictured here is Luis. Not pictured is his adopted sister, Lois.