Outdoor litter box ideas?

I'm building an enclosure for our outdoor cats and I'm stuck on litter box ideas. The enclosure will be their exclusive home; they are not indoor cats.

One idea I had was to build a raised bed like what you'd see in a garden and fill it with gravel and sand. Would that work? What about ways to keep the sand from getting wet? Will it still be usable in the winter? Our temps vary from 20s to 30s in the winter.

Any and all suggestions appreciated!!

@Norachan can likely offer some tips. She has an outdoor enclosure for a small colony of outdoor cats.

Is this enclosure covered at all? You'll want the litter to be covered from the elements and protected from wild animals (raccoon skunks wolves etc) so they can't ambush the cats as they use the toilet.

The enclosure will be built with wood and wire with a wire top so no wildlife will have access to the cats and the litter box will be in an area with a solid roof