Outdoor Cat Enclosure, 3-in-1 Catio Outdoor Cat Enclosure with Tunnel for Outside and Indoor, Cat Tent Portable Catio Pet Playpen with Storage Bag for Kitty and Small Animals

Just because you have a huge space to work with doesn't mean that you have to enclose the whole space from the outset. Perhaps you could start small and just focus on the covered area where you want to put the litter box? You could even add a simple attachment like this to give the kitties extra space without having to construct a huge enclosure at the start.