When I repainted my living room recently, I was worried about paint spilling from the roller tray. I'd bought a tray that was a little larger and deeper because it meant being able to paint longer between refills. But when I hung the tray from the small shelf on my step ladder, it sagged and nearly dumped paint on the floor.
The ladder needed a larger platform to support the tray so the tray sits level. To solve the problem I built a platform from 3/4"-thick plywood as shown in the Exploded View above. It's a couple inches both wider and longer than the paint tray to help catch stray drips. I also added some 1/8" x 1" banding around the edge to contain slightly larger spills that escape the tray.
The tray's feet hook under a notched cleat fastened to the top of the platform. Three screws hold this cleat in place.
To create a storage space for the roller or brushes while refilling the tray with paint, I screwed a plastic drywall mud pan to the side of the platform. This also comes in handy for holding small parts when installing light fixtures or a ceiling fan.
The platform is fastened to the ladder shelf by a carriage bolt and a cleat that spans the support arms on the shelf. A threaded knob holds the cleat tightly in place.