I know it's not what you want to do but you may have to. Have one box per cat. Move at least one of the boxes back upstairs, even if you try it in a different location. Is there a corner or closet somewhere you can put one in? Like mentioned before they have some great box covers that look just like furniture.

I too keep one of the boxes in a closet like catnapper. I have stoppers so that they can't get locked in or out. It works great, it keeps the smell down AND the dogs can't get to it. (But my dogs are big dogs).

Another idea you can try is to put the litter boxes exactly where they were when they used them & slowly move them to their new destination. This mayl take weeks or months (depending on the distance & how the cats adjust) but it may be worth a try.

I oh so want to move the litter boxes but this is the reason I haven't.