Step 1: Buy Your Cat Litter
Most of the work is done by the cat litter manufacturer, it's hard to mold your own plastic bucket. In my experience, use one of the buckets that comes with a full lid, not a screw on cap. As well, square sides makes it much easier. My preference is Scoop-Away, but that's more for their quality litter than anything else.
Cut 2 pieces of pipe strap 8 large holes long. Well, 4 if you want to make 2 buckets.
Gather 8 bolts and matching nuts for each bucket. Ideally they should fit snugly through the holes in the pipe strap.
Pannier hooks, whatever you find works best. I have tried a lot and get the best results from 1 3/4 inch L brackets pounded into shape with a hammer.
Eyebolt and corresponding nuts and washers
A drill, screwdriver, and blade makes the work go quicker
For best results, leave a PBS cooking show on in the background while you work.