All I know is that my 2 cheeky boas are now so used to having a hay box each that they do use it like a loo predominantly because their beloved hay is in there of course!
What I do is use 2 old IKEA shallow plastic storage boxes with first a layer of newspaper, then a fragrant sprinkling of wood shavings followed lastly by a luxurious covering of finest hay. They love it and I know they use these 2 areas the most as the newspaper that covers the base of their enclosure underneath the fleece, is hardly ever stained.
They're not trained, they just want the hay and dive in and out whenever they are inside- so it works as it saves me the effort of doing anything much outside this area. So really as long as you provide hay or limit it to a specific area they will almost certainly pee there. There's no way you can confine the poop though! That's asking too much