I hate to break it to you but nothing you do will be right for The Fur OverLords 
Ours started with three trays (2 cats) but they decided they liked the same one
We put the trays in the under stair cupboard with a catdoor ...but they peed on the floor between the trays .
When my DC were cat sitting we moved the tray into the hall , they were happy . Bought them a deep covered tray . Fine .
They like it cleaned asap and are very noisy (scraping the inner walls)
I was away for a week, when I came home they started soiling the carpet -admittedly it was behind a box propped against the wall , so a 'tunnel' . I think they were protesting at me being back.
They pee at the door end and pooh at the back . Pasr couple of days they've been peeing on the floor . I think body in the tray but their bum hanging out rather than deliberatly peeing on the cat mat .
No other cats in the house , they have a microchip door and get on well together .
They will run inside from the garden to use the tray then go out "You might want to deal with that" look .
So..... they are contrary little buggers . You need to secind guess them.
We use Cat Best litter which is soft and if they bury , doesn't smell .
Plan to go back to a massive tray (flat base from the old guinea pig cage ) and a cardboard box hidey cover .