I'd never heard of a sifter litter tray so Googled it, it's rather large and as you say expensive but it seems to be quiet a clever idea, can I ask what you don't like about it and why it didn't it work?
Well, I prepared the tray with the appropriate clumping litter, and I put a sprinkle of crystal litter on top, cos I know he likes that. The first time he used it with no problems but subsequently, he would not even go out of the window to get to it, so he pooed and peed indoors, hence the use of Dettox, steam and white vinegar. So, I started putting him outside and ignoring his cries to come back in and he started coming back in but refusing to go out, crazy cat! Now, he is coming and going under his own steam, but he won't set foot inside the tray. I think it moves slightly when he stands in it and that makes him panic. Presently, I have pulled the drawer from under the main tray out and he seems to be using that, even when he is missing it with poo, but that is forgiveable and as it is outside, not smelly, easier to clean up too!
I do feel for you all with your cat worries, Sidney isn't seven years of age yet (not until June), so it isn't senility, yet I hope, he's had bouts of cystitis, thought this might be that, but I've been watching him pee (the things we do for our fur babies) and it seems to be a full bladder, no blood in the urine, little or no straining, so I am guessing it was a stress and anxiety related problem, something spooked him and he was too frightened to go out and the poor daft thing couldn't hang on, bless him. Cue a fortune on feliway spray and catnip spray, which I guess wasn't wasted as he now uses the window to go in and out. I will feliway around the flat again today. I am knackered, in pain from the dental stuff and sleepless from coughing so I need this problem with the baby boy like a hole in the head!