I had seen commercial indoor dog "potties" that consisted of some sort of tray/pan, with either some kind of mesh floor or artificial turf, that were designed to allow your dog to relieve himself indoors. The mesh floor or artificial turf provides a pourous substrate to allows your pet to keep his/her feet dry, and the tray/pan collects the urine underneath. However, most had significant (at least from my point of view) price tags, considering the simple technology behind such a product. I knew I could duplicate this general idea, but for a much reduced price tag.
A shallow "underbed" sort of plastic storage tub was a logical urine collection system, but I thought for several weeks about what might make a good "top" that was pourous enough to allow urine to flow through, but with small enough "holes" that my dog could walk comfortably on top and not catch his feet/toes/claws. I considered grid-like flourescent light diffusers that had approximately 3/4" square openings, but these seemed rather flimsy and lightweight for my nearly 20 pound dog. I considered commercially-available plastic kennel flooring ( see http://www.petamenities.com/kennel-flooring.htm ) that was designed for this purpose, but would have involved paying for shipping to my home.
Then, one day in the bread section of the local grocery store, I saw a product that would work VERY well, was readily and locally available, and was both quite pourous and very strong: the sliding "trays" that fit into vertical wire "racks" that are used to transport bread and buns from the bakery to the grocery store. These are made of the same kind and grade of plastic as are plastic milk crates, and are perforated with numerous small diamond shaped holes that are small enough to allow my dog to walk on the surface but allow urine to flow through into a plastic tub below. Being made of plastic, such a tray would be easily trimmable to fit the size of my plastic tub below. Such trays are standard sized to fit into the wire "racks" used on the bakery trucks and are approximately 24.5" X 24.5" square.
I went to my local bakery and simply asked if they had a damaged rack - they gave me one without further question (if it was damaged, I couldn't see it; but I noticed that it was from a different bakery company).