This monster box is 30"x20" of usable pooping space with a flat base that won't collect pools of urine beyond the reach of your scoop. The project shouldn't take more than an hour (unless you choose to spraypaint), and only requires a saw or cutting wheel to get through the polypropylene. If you have a cat capable of leaping, you can just buy the bin and be done with it. Otherwise, I'll show you how to cut the plastic and cover it so your pet's tummy doesn't get caught on the raw plastic edge.
A litterbox should be unobtrusive, easy to maintain, and easy to use. This one is all of the above, and I added a spray-painted stripe across the front to indicate to the puppy where the box began. (You can skip that if your pet is a chewer and you're afraid it'll suffer from munching on spray paint.) The clear edge was confusing to my dog, so I added some contrast. Dogs can't really discern red, but differentiating between concrete and the box was apparently tough when the box was the same color as the ground. The red stripe is just a boundary indicator, and it's totally optional. So optional, in fact, that I didn't document it.