I use Yesterday's News (the soft-textured version only), mixed with a bit of
Feline Fresh Pine Pellet litter, and it works fabulously. My cat, like your Clancy, prefers pellets (I offered him a litter box buffet of different types many years ago and he always went in the box with the pellets). Whenever I hear someone say they have odor problems with their litter box, I suspect they aren't scooping it out often or thoroughly enough.
Where is the location of Clancy's box? Could it be that it's just too far away by the time he realizes he has to use it? (and since the duvet worked so well the first time, well heck, why not go there instead of treking ALL the way down to the laundry room or wherever - you know how "kids" think
). I would put another box or 2 in different parts of the house for the time being. You can eventually move them ever so gradually to somewhere that you'd prefer, but because Clancy is young and this is a new environment for him, the easiest you can make his bathroom access, the better.