We really like the ModKat litter box, but at $200, it's a bit silly... and true to the name (RedneckModern): if you can make it, why buy it?
With Marty, we used Ikea's Sluggis boxes for litter boxes, but they've been discontinued. They were nice as they were truly waterproof/leakproof and looked a bit more sleek that your typical litter pan. In looking for a replacement, I noticed the Trofast series of sliding drawers and thought these might make a nice replacement... and at $3, a bargain.
There's an IkeaHack in here somewhere -- perhaps taking the 3-wide Trofast frame and making it a 3-cat litter station, but the idea of the box sliding like a drawer was enough to get the gears turning. A few dollars in particleboard later -- and the first use of the dado blade on the saw -- the first prototype emerged.
The idea of a top-entry litter box is for the cat to shake off litter while jumping out. For this, a lowered platform open on the face and back serves well -- for the final, I might even put a small square of coir matting to trap more debris or use a perforated material.
The final will eventually be made of solid wood or plywood and should cost well under the ModKat price of $200 for the materials. I've also noted that the bottom is kind of unnecessary as the tray is rigid enough to support itself (and cat) while locked into the dado groove. I think walnut plywood and a floating green tray might look pretty cool. Maybe with a porthole on the face or side, too.